PLEC TO G N A TH I.
F amily.—T ETRO D O N TIDÆ .
1 . D i o d o n n y c t h e m e r u s . C u v .
Diodon nyctliem enis, Cm. Mem . du M us. tom . iv. p. 135. pi. p.
A species of Diodon in Mr. Darwin’s collection, the number attached to
wliich has been lost, and of which the locality is in consequence unknown, appears
referable to the D . nycthetnerus of Cuvier.
The spines are long, measnring three quarters of an inch in length; round, sharp, and not
very close together. There are five in the front row between the eyes, seven in a transverse
row between the pectorals, and ten or eleven between the snout and the dorsal in a longitudinal
one ; none exactly on the upper part of the tail, but one on each side of the base of it, a
httle below the termination of the dorsal fin, and a corresponding pair still lower down. The
spines on the belly are shorter, and rather closer together than those on the back. One of
those on the back in this specimen is accidentally forked.
The true teeth appear on the surface of the jaws like minute scales,of the genus Scarus. as in several species
The fin-ray formula is as follows :
D. 13 ; A. 13; C. 9; P . 20.
Length 5 inches 6 lines.
The colours, so far as can be judged, the specimen being in spirits and not in
very good condition, answer to Cuvier’s description of them with tolerable exactness.
2 . D i o d o n r i v u l a t u s . C u v .
Diodon rivulatus, Cuv. M em . du Mus. tom. iv. p. 129. pi. 6,
-4n individual apparently of this species was picked up by Mr. Darwin on the
shore of the Rio P lata at Maldonado. It agrees with Cuvier’s description, excepting
that the undulating lines are not visible, probably owing to the state of the
specimen when found.
The spines are short, barely a quarter of an inch in length, but very strong, compressed,
and resembling canine leeth. There are three in the first row between the eyes; about six in
a transverse row acro.ss the back, and seven or eight in a longitudinal one. Beneath they arc
shorter and more nnmerons. Tho orbits are elevated in ridges, and project forwards over the
eyes Two very small barbules attached to the lower lip. Surface of the jaws smooth, the teeth not appearing as scales.D
. 11 ; A. 1 0 ; C. 8 ; P. 22.
Length 5 inc. 3 lin.
As Cuvier observes, the D . geometricus of Bl. and Schneid.* approaches very
closely this species, and I can hardly think it to be distinct. Yet neither in Mr.
Darwin’s specimen, which in all other respects agrees exactly with Schneider’s
figure, do I discern any appearance of the hexagonal meshes on the surface of
the body.
3 . D i o d o n a n t e n n a t u s . C u v .'!
Diodon antennatus. C m . M em . du M us. to m .iv . p . 131. pi. 7-
A third species of Diodon, brought hom ehy Mr. Darwin, and taken by him at
Bahia, in Brazil, is either the young of the D . antennatus of Cuvier, or else new ;
hut the only individual in the collection is quite small, and not more than an inch in
length, excluding caudal. The flesliy filaments above the eyes, which, according
to Cuvier, so peculiarly distinguish the D . antennatus, are very distinct,—but I
see none on the sides. The grouiid colour would seem darker than he describes,
so as to render the spots and markings on the upper parts not distinguishable
from it now, if they ever existed. In spirits it appears of a nearly uniform deep
brown red. The spines, or rather papillte, are also shorter than represented in his
figure ; hut this may he only the effect of immaturity.
According to Mr. Darwin, the colours when recent were as follows ;—“ Above
blackish brown, beneath spotted with yellow. Eye with the pupil dark blue ; iris
yellow, mottled with black.” It is added “ On the head four soft projections ;
the upper ones longer, like the feelers of a snail.
Mr. Darwin observes, “ that the dorsal, caudal, and aiial fins, in this species,
are so close together that they act as o ne: these, as well as the pectorals, are in a
continued tremulous motioii even when the fish is otherwise motionless. The
animal propels its body hy using the posterior fins in the same maitner as a boat
is sculled, that is, by moving them rapidly from side to side with an oblique surface
exposed to the water. The pectoral fins have great play, which is necessary
to enable the aiiiiiial to swim with its back downwards.”
Mr. Darwin made some further observations on the habits of this species,
which have already appeared in his “ Journal,” to which I may refer the reader.f
The tendency of tliem is to explain the process hy which the water and air are
absorbed, when the Diodon distends itself into a spherical form ; and to show that
the fish can swim, when floating in this state with its back downwards, which
Cuvier doubted. He tliiiiks that the water is taken in partly for the sake of
regulating its specific gravity. He also notices a curious circumstance with
respect to this species, viz., “ that it emitted from the skin of its belly, when
handled, a most beautiful carmine red and fibrous secretion, which permaneittly
stained ivory and paper.”
• Sgst. Ichth. ^\. a s. t p p . 13, I* .