LIASSIC FORMATIONS. 
85 
Reptiles ; and, in some, is larger than m Ichthyosaurus} it has been continued on 
to modern Lacertilia, but has become obliterated in the Crocodilian order. Both diapo- 
physes and parapophyses^ 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 Muberculum’ (PI. XXI, fig. 2 , «, l). 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 Ichthyoyjterygia. The teeth in this order show a trace of the older Labyrin- 
thodont cliaracter in the converging folds of cement penetrating their base,^ 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 teetli 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 Ichthyosaurian species (PL XXX, fig. 3 ), five in 
number. The scapular arch (PI. XXVIII, fig. 4 , Ich. latimanm, Ich. communis, e. g.), 
includes an episternum (45) 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, are 
supported by iliac, ischial, and pubic bones. The terminal ctuidals 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 tlie Fossil Tieptilia 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'; Bicynodon, Ptychognathns, Oudenodon, Kisticephaliis, pis. Ixiv, Ixv; Procolophon, pi. xxii, 
figs. 4, 8. 
2 Op. cit., Pareiamurus, pi. x, figs. 1, 3, d, p ■, Bicynodon, pi. lii, fig. 1 ; p!. liii, fig. 3, d. 
® A transverse section of the base of the tooth of an Ichthyosaur gave the first clue to the structure 
of that of the Lahyrinthodon. ‘Odontography,’ 1840, p. 201, pi. Ixiv B, fig. 3. 
