242 HISTORICAL PALAEONTOLOGY. 
of true Sharks (JVo/zdanus) occur for the first time ; but by far 
the greater number of remains referable to this group are still 
the fin-spines and teeth of “ Cestracionts,” resembling the 
living Port-Jackson Shark. Some of these teeth are pointed 
(Hybodus) ; but others are rounded, and are adapted for crush- 
ing shell-fish. Of these latter, the commonest are the teeth of 
Acrodus (fig. 175), of which the hinder ones are of an elon- 
gated form, with a rounded 
surface, covered with fine 
transverse striz proceed- 
ing from a central longi- 
tudinal line. From their 
general form and striation, 
Fig. 175.—Tooth of Acrodus nobilis. Lias. and their dark colour, these 
teeth are commonly called 
“fossil leeches” by the quarrymen. 
The Amphibian group of the Labyrinthodonts, which was so 
extensively developed in the Trias, appears to have become 
extinct, no representative of the order having hitherto been 
detected in rocks of Jurassic age. 
Much more important than the Fishes of the Jurassic series 
are the Lefties, which are both very numerous, and belong to 
a great variety of types, some of these being very extraordinary 
in “their anatomical structure. The predominant group is that 
of the “ Enaliosaurs” or “ Sea-lizards,” divided into two great 
orders, represented Tespectively by the Lchth LyOsaurus and the 
Plesiosaurus. 
The Lchthyosauri or ‘ Fish-Lizards” are exclusively Meso- 
zoic in their distribution, ranging from the Lias to the Chalk, 
but abounding especially in the former. They were huge 
Reptiles, of a fish-like form, with a hardly conspicuous neck 
(ig. 176), and probably possessing a simply smooth or 
Fig. 176.—lchthyosaurus comimunis. L.ias. 
wrinkled skin, since no traces of scales or bony integumentary 
plates have ever been discovered. The tail was ‘long, and 
was probably furnished at its extremity with a powerful ex- 
pansion of the skin, constituting a tail-fin similar to that pos- 
sessed by the Whales. The limbs are also like those of Whales 
