LIASSIC ICHTHYOSAURS. 43 



They are conspicuous Ichthyopterygian characters, and are associated, as far along the 

 spine as they are distinctly developed, with the double-jointed ribs, showing ' capitulum ' 

 and ' tuberculum ' (Tab. XVII, fig. 2, a, b). The prezygapophyses of the atlas converge, 

 descend, and aid in forming the anterior cup, which receives a corresponding 

 convex joint-surface of the occipital vertebra ; the change from the double condyle of 

 the oldest air-breathing Vertebrates to the single condyle in Triassic Reptilia is retained 

 in the IchthyoiJterygia. The teeth in this order show a trace of the older Labyrin- 

 thodont character in the converging folds of cement penetrating their base,^ but the 

 alveolar partitions of their native groove ai'e not complete in any part of the tooth-bearing 

 tract. Anchylosis of the tooth-root to the jaw, seen in Mosasauroids and modern 

 Lizards, is not effected in any Ichthyosaur. The teeth retain this freedom, as in 

 Crocodiles, with a similar repeated succession and Shedding ; as in Crocodiles, also, they 

 are confined to the maxillary, premaxillary, and premandibular (" dentary ") bones, but 

 with the ordinal character of much greater length of the premaxillaries than of the 

 maxillaries. The orbits, in Ichtliyo^iterygia, are conspicuous for their size; the circle 

 of sclerotic plates usually found fossilised in them exemplifies a primitive vertebral 

 character under a modification continued on in Chelonia, Lacertilia, and Aves. The 

 nostrils are distinct, and antorbital in position. The limbs are natatory, with many- 

 jointed digits, and these exceed, in some Ichthyosaurian species (Tab. XXVI, fig. 3), five 

 in number. The scapular arch (Tab. XXIV, fig. 4, Ich. latimanus, Ich. communis, e. g.), 

 includes an episternura (46) and clavicles (53), with a well-developed coracoid (52) and 

 scapula (51), the latter near to, but detached from, the occiput. The hinder part of the 

 vertebral column is as free for natatory work as in Whales ; there is no sacrum, but 

 a pair of pelvic fins is constant, and these, usually smaller than the pectoral ones, ar"^ 

 supported by iliac, ischial, and pubic bones. The terminal caudals are modified for thi 

 support of a tegumentary fin, but are compressed, not depressed, the fin being vertical, 

 not horizontal. 



The adaptive modifications of the Ichthyopterygian skeleton, like those of the 

 Cetacean, relate to their medium of existence; they are superinduced, in the one 

 case upon a Reptilian, in the other upon a ]\Iammalian type, and both show analogies to 

 the Vertebrates which the " waters first brought forth." But that the Ichthyopterygians 

 did not breathe by means of gills is shown by the absence of the branchial framework, 

 and by the presence, position, and structure of passages leading from the nostrils to the 

 palate for the course of currents of air on their way to lungs, which were protected and 

 worked by movable thoracic-abdominal costal girdles. Herein these old Sea-reptiles rise 

 higher in structure than some modern cold-blooded air-breathers, such, e.y. as Batrachians 

 and Chelonians. 



An Ichthyosaur by the shortness, one may say absence, of neck, and equality of 



1 A transverse section of the base of the tooth of an Ichthyosaur gave the first clue to the structure 

 of that of the Labyrinthodon. 'Odontography,' 1840, p. 201, pi. Ixiv b, fig. 3. 



