of th e cartilage passing wedge-like between th e processes and its backward motion being arrested by
its wedging in between them. Tha t p a rt of th e mesethmoid th a t lies on either side of th e median keel,
forms p a rt of th e floor of th e nasal pit, b u t th e nasal sac rests h u t litfle upon it, being crowded off
it by th e rostral. On the dorsal surface of th e posterior portion of the mesethmoid, on either side,
th e frontal rests, th e articulating edges of the two bones being often more or less dovetailed together.
The frontal of either side extends forward to th e base of th e corfesponding mesethmoid process, th e
two frbntals entirely covering th e posterior p a rt of the mesethmoid excepting only a small median
portion.T
he mesethmoid of Scorpaena, although undoubtedly a so-called primary bone, consists of
two distinctly different portions. One of these portions is a th in dense layer of superficial bone. The
other portion is a deeper one, of quite different appearance, which underlies the central portion only
of th e superficial portion, and there replaces portions of th e cartilage of th e skull. I have made no
a ttem p t to study th e development of these two components of th e bone, or even to determine, other
th a n most superficially, the ir character. The investigation has however involved th e examination
of several series of sections of specimens from 45 mm to 55 mm in length, and in these sections th e
condition of several of th e cranial bones was more or less carefully noted. In these specimens the
deeper portion of th e mesethmoid of th e adult, the portion th a t replaces pa rts of th e cartilage of the
skull, has not y e t begun to develop. The superficial portion of the bone is represented by a th in plate
th a t lies closely upon th e cartilage of th e skull, without intervening membrane, and must be primarily
wholly of perichondrial origin; b u t this perichondrial plate receives, a t certain places, accretions, or
additions, to its outer surface, and these accretions, although they p resent in sections exactly the same
appearance as th e perichondrial plate, seem to be of purely membrane origin. This is particularly
noticeable, in my specimens, along th e lines of th e articulation of th e mesethmoid with th e frontals,
and in th e mesethmoid processes. These la tte r processes rise from and are directly and unbrokenly
continuous with th e layer of perichondrial bone, b u t they are so important, and so wholly o u t of all
relation to existing cartilage th a t th ey must be largely of purely membrane origin. Otherwise, there
must b e here an important fold in th e perichondrial tissues which occupies th e place of a cartilaginous
process found in other earlier forms, and this tissue must give origin to perichondrial bone without
th e related development of any cartilage.
The superficial portion of th e mesethmoid of the adult, of dermo-perichondrial origin, as above
explained, is in direct contact, on either side, with a corresponding portion of the ectethmoid. Anteriorly
it is in contact w ith th e dorsal limb of th e vomer. The deeper p a rt of th e m esethmoid, of endosteal
origin, is every where in contact w ith, and replaces portions of th e antorbital cartilage, and does n o t
extend ventrally through th a t cartilage.
E C T E T H M O I D .
The ectethmoid, on either side, is th e prefrontal of many authors, the parethmoid of Swinner-
ton’s descriptions of Gasterosteus, and th e preorbital ossification of my own descriptions of Amia and
Scomber. I t has, in Scorpaena, a dragon-wing-like appearance, and consists of a wing, an arm th a t bears
th a t wing, and a small b it of shoulder or body th a t bears the arm and supports the mesial edge of the
wing. The wing is a th in plate of bone, concave posteriorly and convex anteriorly, which projects upward
and backward, in a curved line, from the dorsal surface of the arm, and is fused in th e basal portion
of its mesial edge with a p a rt of th e body of th e bone. The wing of th e bone develops without being
preformed in cartilage, resembling,, in this, th e mesethmoid processes of the mesethmoid; but, as in
th e case of th e la tte r processes, th e bone is perhaps of perichondrial origin, developed, in th a t case, in
perichondrial membrane, b u t without th e related development of cartilage. The arm of the bone
projects antero-latero-ventrally from a relatively small ossification of the side wall of th e skull, this
ossification lying immediately in front of the o rbit and forming the body of the bone. From the ventro-
mesial corner of th e anterior surface of th e wing, where the arm and wing join th e body of the bone,
two ridges start, and diverging slightly run upward and backward to the dorso-posterior edge of the
wing. The lateral one of these two ridges terminates in th e preocular spine, which lies near the dorsal
end of th e lateral edge of th e entire bone. The mesial ridge does not terminate in a spine, b u t gives
support, along its mesial surface, to th e lateral edge of the anterior end of the frontal, a pronounced
ridge, formed b y the two bones, here appearing on th e dorsal surface of th e skull. The curved lateral
edge of th e wing tu rn s sharply forward, a t its ventral end, to join th e outer end of the arm of the
bone, a sharp corner, b u t n o t a spine, marking this angle in the edge of th e wing. In certain of the
Scorpaenidae there is said to here be a short, b lu n t spine. On the dorso-anterior surface of the wing,
lateral to the preocular spinous ridge, there is sometimes an eminence, or short spine, which occupies
the position of a stout process found on the bone of Cottus, th a t process there giving support to a
lateral process of th e nasal bone, as will be later described.
The a rm of th e ectethmoid is a stout, flat, quadrant-shaped process, which has a slightly curved
o u te r edge presented ventro-laterally, and nearly straight dorsal and mesial edges which are both
considerably thickened. The entire arm, in the adult fish, thus looks like two stout processes th a t
arise from a single point, and, diverging, are connected by an intervening portion of thinner bone.
In 45 mm specimens the two process-like portions of th e arm are of cartilage, enclosed in perichondrial
bone, th e thinner intervening portion being of bone similar to th a t th a t forms th e wing of the bone.
In the adult, one of th e process-like portions of the arm lies in a nearly horizontal position, directed
laterally and slightly forward. I t forms th e dorsal edge of the entire arm, and its dorsal surface forms
p a rt of th e dorsal surface of the entire skull. Its anterior surface is slightly concave and forms the
latero-posterior and p a rt of the posterior wall of th e nasal pit. Its outer end is considerably thickened
and forms a large articular head, capped with cartilage, which is presented latero-ventrally and but
s lightly anteriorly, and gives articulation to an articular facet on th e dorsal edge of the lachrymal.
The other process-like portion of the arm forms no p a rt of the bounding walls of th e nasal pit. I t is
•directed almost directly ventrally, and its ventral end forms a small elongated articular head, capped
with cartilage, which gives articulation to th e posterior ethmoid articular surface of th e palatine.
Between these two articular surfaces, the outer edge of the arm of th e ectethmoid is thin, slightly
•concave, and not capped with cartilage.
The body of th e ectethmoid is an ossification of the ventro-lateral corner of the antorbital
process of the chondrocranium, and consists, as th e mesethmoid does, of a superficial layer of dense
dermo-perichondrial bone, which overlies, b u t projects everywhere beyond, a deeper endosteal portion
which replaces portions of the cartilage. This body of th e bone is of less important dimensions than
the p a rt th a t I have described as its arm. I t extends b u t slightly posterior to the point of origin of
th a t arm, b u t its anterior portion projects considerably anterior to the arm, is gutter-shaped, and
embraces the thin lateral edge of this p a rt of the a ntorbital cartilage. The rounded lateral surface of
this anterior portion of the body of the bone is slightly concave, longitudinally, and a deep rounded
angle is thus formed between itself and the concave anterior edge of the horizontal, process-like portion