106 
FOSSIL REPTILIA OF TliE 
All are enveloped in one sheath of smooth integument, which is continued beyond the 
framework to an obtusely pointed end, and likewise extends some way beyond the 
hinder or ulnar border of the bones, where it seems to have been supported by numerous 
fine cartilaginous rays unless the appearance be interpreted as due to parallel bifurcate 
wrinkles of the skin (PI. XXVIll, fig, 5).^ 
The ilium (PI. XXX, fig. 5, 62) is an oblong, subcompressed bone, slightly enlarging 
at the acetabular end to form the articular surfaces for pubis, ischium, and femur; some- 
times, also, more slightly expanding to the opposite free flattened end. This part, if 
attached to any of the vertebrae, would be so by ligamentous or soluble tissue. The length 
of the ilium does not exceed, usually, that of three contiguous centrums. 
The ischium (ib. 63), of minor length, is thickened at the acetabular end, which shows 
facets for the ilium, the pubis, and its share in the shallow styloid cavity for ligamentous 
attachment of the head of the femur. The body of the ischium, expands, more or less in 
different species, to its free non-articular border, anterior to which it unites synchondro- 
sally with the pubis. This (ib., 64) is usually a somewhat longer bone ; it also expands 
to join the ischium at its lower end ; the opposed borders of the two bones are concave, 
and inclose a long and large ‘ obturator ’ vacuity. 
Modifications of these pelvic elements are shown in Ichthyosaurus communis and 
Ich. latifrons, and will be noticed in some other species. 
The femnr (PI. XXXI, fig. 1, 6o) resembles the humerus, but is smaller and usually 
rather less broad. Its distal end supports two cnemial bones, repeating the shape and 
relative proportions of the antibrachial ones. To these succeed three tarsal ossicles, which 
support four series of digital ossicles. But in Ich.. communis the anterior or tibial series 
soon bifurcates, and supplementary ossicles are applied to those of the fibular or hinder 
digit. The shape of the ventral or pelvic fin thus supported is similar to, but relatively 
less broad (in Ich. communis) than, the pectoral one. 
The rare illustration given in Plate XXVIII, fig. 5, is of a pelvic fin. 
(I. Osteological Summary. — At the risk of some repetition the following remarks, 
the result of such comparisons as I have had time or means of extending over the cold- 
blooded air-breathers, may not be unacceptable. The comparison with existing Beptilia 
is almost restricted to the Lacertian and Crocodilian modifications. 
In Crocodiles the exoccipitals leave an intervening tract of the basioccipital, which 
thus takes its share in support of the ‘ medulla oblongata’; in Lizards the exoccipitals meet 
upon the basilar element, as in Ichthyosaurus, but the extinct Reptile differs from both 
in the exclusive formation, by the basioccipital, of the condyle articulating with the 
atlas. Such joint in Fishes is afforded by the basioccipital only, but the articular surface, 
as a rule, is concave instead of convex. The superoccipital is excluded from the foramen 
^ ‘ Transactions of tlie Geological Society,’ 2nd series, vol. vi, p. 199, pi. xx. 
