536 MULLER ON THE STRUCTURE OF THE GANOIDS, 
maxillary bone, and laterally by the upper jaw. They have a 
czecum, and their anus is situated beneath the throat. The clas- 
sification of the Physostomi now rests on solid foundations ; but 
we must not conceal the fact, that the families of the Acantho- 
pteri, in which the characters pointed out by Cuvier are mostly 
retained, still present many artificial distinctions. 
Plectognathi, Cuy.—Although the immoveable articulation of 
the upper and lower jaws in this order is not constant, and this 
structure likewise occurs in other fishes, as in several Characini 
(Serrasalmo, &c.), still Cuvier’s Plectognathi have considerable 
affinity in their cutaneous tegument, the scales, roughness, spines, 
and (osseous) plates of which differ from ordinary fish-scales. 
To them belong the families, Balistini, Ostraciones and Gymno- 
dontes. 
The Lophobranchii form the last order of the Teleostei, which 
do not essentially differ from the vertebrate fish generally. 
The Selachii form two orders, the Plagiostomi and the Holo- 
cephali. But the Plagiostomi must be again divided. into sub- 
orders, the Sharks and Rays; for the Rays are distinguished 
from the Sharks by the perfect annular clavicle extending as far 
as the under surface of the skin of the back; by the gill-cavities 
being fissured inferiorly ; the absence or adhesion of the eye- 
lids ; the union of the pectoral fin with the head; by the cartilage 
of the cranial fins ; and what occurs in all the Rays, the coalition 
of the anterior part of the back-bone to a single large cartilage 
without any vertebral division, as also occurs in Pristis ; whilst 
the Saw-fish among the Sharks (Pristiophorus} resemble the 
Sharks in that as in all other respects. The families of the Sharks 
are—Scyllia, Nyctitantes, Lamnoidei, Alopecie,Cestraciones, Rhi- 
nodontes, Notidani, Spinaces, Scymnoidei, and Squatine. Re- 
garding the families of the Rays, as arranged in the system of 
the Plagiostomi, we need only remark, that the genus Platyrhina 
belongs to the oviparous Rays, hence to the family of the Raie*. 
* See Abhand. d. Akad. d. Wissenschaft a. v. 1840, p. 246. The relations 
of the genus Z7rygonorhina, with the ova of which I am unacquainted, are pro- 
bably the same as those of Platyrhina. 
