to think is not strictly correct, for the fossa in some of these la tte r fishes is certainly similar to th a t
in Scomber, which, as above explained, is something more th a n th e equivalent of th a t in Sclerognathus
and Diplophysa. Sagemehl further says th a t th e fossa in th e fishes above mentioned has arisen wholly
independently of th a t in Sclerognathus. This also seems to me incorrect. Sagemehl himself says th a t
th e fossa in Amia represents a primitive condition. From th a t fossa, th e fossa of Scorpaena is readily
and directly derived; and from th e fossa of Scorpaena th a t of Sclerognathus would be directly derived
b y th e simple reduction, which has actually tak en place in th e la tte r fish, of th e extrascapular and
suprascapular bones. And from the condition found in either Scorpaena or Sclerognathus th a t in
Scomber would be produced b y th e continued anterior prolongation of th e muscles th a t fill the groove,
th e muscles passing dorsal to th e parietal and frontal bones instead of v entral to them. If the muscles
passed beneath th e parietal and frontal i t would give rise to the conditions found in Macrodon and
Elops. From th e condition found in Sclerognathus, also, th a t in Nemachilus, Misgurnus, Cobitis and
Acanthophthalmus would be derived by a simple constriction of the groove, due to th e encroaching
ingrowth of the pterotic and epiotic. I t seems almost unnecessary to s tate th a t th e conditions found
in these la tte r fishes is certainly not a primary one, th e tru n k muscles here being seen in process of
excavating th e fossa in the solid bones of th e skull.
In Gasterosteus, according to Swinnerton (’02), th e temporal groove is shallow and
wholly uncovered, th e extrascapular (supratemporal) being said to lie superficial to th e tru n k
muscles b u t wholly posterior to the hind edge of th e skull. Swinnerton considers the groove in
this fish as similar to th a t in Amia. I t is, on the contrary, similar to, b u t less developed th a n
th e groove in Scomber, in which fish th e groove also lies wholly external to, instead of internal
to, th e parietal. A «imiUr groove is also found in all the Clupeoid fishes described by Ridewood
(’04c), excepting only Chanos, b u t there is, in these fishes, a related temporal foramen n o t found
in Scomber or Gasterosteus.
In Gymnarchus th e temporal fossa undergoes special development and acquires relations to
th e auditory organ, as I have recently shown (’04); and, judging from Ridewood’s descriptions, the
same may be tru e of th e other Mormyridae and of th e Notopteridae. In these la tte r fishes, Ridewood
(’04b) does n o t describe either a temporal fossa or groove, b u t his figures show such a groove, apparently
similar to th a t of Scomber, this groove being more or less roofed by an extrascapular (supratemporal)
bone or bones, and having, in its mesial wall, a large opening. This opening Ridewood
describes as a lateral cranial foramen, and he considers i t (’04a, p. 61) as th e possible homologue of
th e preepiotic fossa of his descriptions of Clupea.
In Gonorhynchus, according to Ridewood (’05 b), there is no suggestion, even, of a temporal
groove; b u t, judging from th e figures given, it would, seem as if there might there be a very shallov
groove of th e kind found in Scomber.
I t may here be s tated th a t, in Notopterus, Ridewood says th a t th e extrascapular (supratemporal)
.,does n o t carry th e sensory tu b e “ , th e supratemporal cross-commissure lying anterior to th a t
bone. Bridge (’00, p. 517) however says th a t this sensory canal of Notopterus is „strengthened“ ,
along its lateral and inner walls, by „two longitudinally arranged, thin, semi-cylindrical bones, or
sensory ossicles“ ; which ossicles must accordingly be, or belong to, th e extrascapulars. Be this as it
may, th e variation seems morphologically unimportant, for th e extrascapular does not actually
„ ca rry “ th e canal in Gymnarchus either, the bone lying wholly superficial to th e canal.
D I L A T A T O R F O S S A .
The dilatator fossa of Scorpaena is a small p it th a t lies directly above th e interval between
th e two articular facets for th e hyomandibular, near th e dorso-lateral edge of the skull. Its posterior
portion is enclosed in th e pterotic, its anterior portion in th e sphenotic. Between these two bones
there is, in th e lateral edge of th e roof of th e fossa, an interval, which leads onto the roof of the
primordial cranium b u t is covered by th e purely dermal postfrontal bone. The fossa gives origin to a
small superficial bundle of th e dilatator operculi muscle, and corresponds to the deeper, posterior
portion, only, of the large dilatator groove of my descriptions of Scomber. If the dilatator muscle
were here, in Scorpaena, to push upward through th e interval between the pterotic and sphenotic,
i t would lift th e postfrontal from th e dorsal surface of th e primordial cranium, and occupying the
region between th e pterotic and postocular spinous ridges, give rise to the dilatator groove of Scomber.
In th e bottom of th e fossa there are two openings. The anterior one opens directly into the
oticus canal and transmits a vein which is associated with the ramus oticus. The other transmits
a vein th a t comes from regions median to the oticus, between the primary skull and the overlying
dermal bones.
The dilatator fossa and groove of th e nomenclature here employed are the lateral temporal
fossa or groove of Ridewood’s recent descriptions of the teleostean skull. B ut Ridewood does n o t call
a tten tio n to the marked difference between these fossae and grooves, as ju s t above set forth, and
doubtless did n o t recognize it. In both Elops and Megalops, for example, the lateral temporal groove
of Ridewood’s descriptions would seem to be th e homologue of th e dilatator groove of Scomber;
while in both Albula and Bathythrissa, the lateral temporal fossa would seem to be the homologue
■of th e dila ta tor fossa of Scorpaena; the lateral temporal grooves and lateral temporal fossae of his
descriptions th en not being equivalent.
In th e Clupeidae, there would seem to be, from Ridewood’s descriptions, a dilatator groove
similar to the groove in Scomber.
M E S E T H M O I D .
The mesethmoid has a keel-shaped anterior portion* the keel directed dorsally, and a thin
tapering posterior portion which is concave longitudinally, on its dorsal surface, and ends posteriorly
in a point. Between thesé two portions, and a t about the middle of the length of th e bone, two stout
prong-like processes arise, one on either side, and project upward and slightly laterally and forward.
'The hind edge of each process is usually slightly convex, curving upward, forward and laterally,
while the ventral half of the anterior edge is slightly concave; the whole process having th e appearance
of a horn projecting upward, forward and laterally b u t with the point cut off obliquely, so th a t
a flat surface is presented almost directly forward. This flat surface gives origin to the ethmo-maxil
la ry ligament, while the lateral surface of the process gives support and attachment to th e hind ena
•of the nasal bone. This s tout and prominent process has long been known and is the homologue of
th e quite differently appearing dorso-lateral process of my descriptions of Scomber, a fact I did not
recognize when describing th a t fish. I t has been called the mesethmoid process in certain descriptions
•of the Scorpaenidae, a name which I adopt as much preferable to the term employed by me.
The median keel on the dorsal surface of the anterior portion of the mesethmoid forms the thick
b u t low internasal wall of the skull. I t extends backward a varying distance between the two mesethmoid
processes, and on it the cartilaginous rostral slides backward and forward, the tapering hind end