LIASSIC FORMATIONS. 85 



Reptiles ; and, in some, is larger than in Ichthyosaurus} whence it has been continued on 

 to modern Lacertilia, but has become obliterated in the Crocodilian order. Both diapo- 

 physes and parapophyses 3 appear in Reptilian vertebrae at the same geological period, and 

 are carried on in the Crocodilian modification of the class, but are lost in existing Lacertilia. 

 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' (PI. XXI, 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 Ichthyopterygia. The teeth in this order show a trace of the older Labyrin- 

 thodont character in the converging folds of cement penetrating their base, 3 but the 

 alveolar partitions of their native groove are 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 Ichthyopterygia, 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 lchthyosaurian species (PI. XXX, fig. 3), five in 

 number. The scapular arch (PI. XXVIII, fig. 4, Ich. latimanus, Ich. communis^ e. g.), 

 includes an episternum (46) and clavicles (58), 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, are 

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

 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 



1 ' Descriptive and Illustrated Catalogue of the Fossil Reptilia of South Africa in the British 

 Museum,' 4to, 1876 ; Galesaurus, pi. xviii, fig. 8, 7'; Petrophryne, pi. xx, fig. 18, 7-; Gorgonops, pi, xxi, 

 fig. 3, 7'; Dicynodon, Ptychognathus, Oudenodon, Kisticephalus, pis. lxiv, ixv ; Procolophon, pi. xxii, 

 figs. 4, 8. 



2 Op. cit., Pareiasaurus, pi. x, figs. 1, 3, d, p ; Dicynodon, pi. lii, fig. 1 ; pi. liii, fig. 3, d. 



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

 of that of the Labyrinlhodon. ' Odontography,' 1840, p. 201, pi. lxiv b, fig. 3. 



