174 GEOGNOSY AND GEOLOGY. 
abundant of the Pycnodonts. These were short and high fishes, with 
a strong skeleton. The eyes lay very high above the rather wide mouth. 
The dorsal and anal fins were of considerable extent, reaching to the 
homocercal caudal: P. rhombus ( fig. 64). ; 
In the family of the Lepidoides, the genus Dapedium is dickens bya 
short and flat body, small head, sina diminutive mouth, cummin with 
sharp curved teeth. The fins are short and weak, the ventral and dorsal 
nearly opposite. The thick rhomboidal scales were covered with enamel, 
and connected by processes. D. punctatum (fig. 65) and D. politum 
( pl. 41, fig. 11) occur in the lias. The present creation exhibits few 
representatives of these fish with angular connected ganoid scales, the 
chief representatives being the North American Lepidostei, of which some 
ten species are known to naturalists. Pl. 41, fig. 12%, represents one of 
these species ; fig. 12%, a portion of its jaw. 
The species of Megalurus and Aspidorhynchus are conspicuous among 
the Sauroids. The former ( pl. 38, fig. 66) have large rounded caudal fins, 
high dorsal, and large pectoral fins. ' Their shape is compact and stout, the 
head short, with a moderately large mouth, provided with thick conical 
teeth. Aspidorhynchus ( pi. 41, fig. 138) was a very long, narrow, and 
cylindrical fish, with a long upper jaw projecting far over the under, both 
being furnished with conical acute and unequal teeth; the scales are 
higher than long, the dorsal fins opposite to the anal, and both very near the 
large forked tail. Both Megalurus and Aspidorhynchus are found in the 
upper Jura strata, the latter also in the cretaceous. Pl. 41, figs. 14-28, 
represent teeth of certain ambiguous fishes. 
The reptiles of the Jura belong especially to the Enaliosaurians, 
represented by the genus Ichthyosaurus and Plesiosaurus. The Enalio- 
saurians are principally characterized by the fish-like vertebree. These are 
flat at both ends, and conically concave ( pl. 42, figs. 24 and 27), forming a 
long vertebral column by their union. The four feet of the animals are 
broad and flat, without either fingers or claws, being thus true paddles. 
The normal bones of the feet are subdivided much more than in recent 
forms. The sharp conical teeth stand in long grooves of the jaws, and each 
one inserted in a distinet socket, as in the crocodiles. The head also exhibits 
some resemblance to these latter animals. 
The Ichthyosaurus (I. communis, pl. 41, fig. 26% from the Lias) has been 
found of a length of 40 feet. The large head possesses a long acute snout, 
furnished Rh 120 to 160 conical grooved teeth ( figs. 26, re and c), inter- 
locking together on closing the jaws. The eyes are very large and circular, 
with a bony sclerotic, composed of several plates such as are now found only in 
birds and turtles. The vertebre are numerous, those bearing ribs amounting 
to as many as 40. The short neck contains only from five to ten vertebre. 
The ribs inclose the whole body, and are connected with a T-shaped 
sternum. The anterior extremities are more powerful than the posterior, 
both probably having been covered with angular plates of horn, while the 
body was naked. Like some recent fishes, the Ichthyosaurus possessed a 
spiral intestinal canal, which impressed the same character on the 
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