LIASSIC FORMATIONS. 
117 
remains of the present species. It seems to have been characterised by long and 
narrow, but powerful fore paddles. 
In the pelvic bones the ilium (PI. XXXI, fig. 1, 62 ) presents a straight, flattened, 
slender form. The ischium (ib., 63 ) is remarkable for its breadth, especially at its 
medial end. The pubis (ib., 64 ) is less expanded there ; its anterior border is straight. 
The femur (ib., 65 ) is longer in proportion to its breadth than the humerus ; its 
proximal end shows a large depression, probably for the insertion of a stout ligament. 
The tibia (ib., 66) presents an anterior emargination, as in the radius (54) ; the same 
character is repeated in the two succeeding ossicles at the same margin of the fln- 
framework. Here, also, but three digital series are preserved, with a few small 
supplemental ossicles along the fibular border of the fin. 
The disposition of the distal ossicles in both pairs indicates that the ligamentous or 
fibro-cartilaginous uniting medium of their framework may have been more abundant 
than usual, allowing greater flexibility of the terminal part of the long and narrorv 
paddles of Ichthyosaurus jjlatyodon. The least incomplete skeleton of this huge species 
in the British Museum, the subject of PI. XXXI, fig. 1, is from an individual of about 
20 feet in length ; but portions of others — the skull, for example, which may be seen at 
the Geological Society’s Booms at Burlington Blouse — indicate a total length of the 
individual so represented, of at least 30 feet. 
The Lias of the Valley of Lyme Regis is the chief depository of Ich. platyodon, but 
its remains are pretty widely distributed in the same Mesozoic zone. They have been 
found in the Lias of Glastonbury, of Bristol, of Scarborough, of Whitby, and of Bitton 
in Gloucestershire. The Ammonites associated with the bones of the subject of 
PI. XXXI, fig. 1, are of the species Arietites semicostatiis^ characteristic of the greyish 
limestone (Lower Lias) of Lyme Regis. 
€. IcHTHIOSAIJRUS LONCHIODON,^ OlO. PI. XXIV, figs. 6, 6' ; PI. XXXI, figs. 4—7. 
This species, which appears to have attained a bulk second only to that of the Ich. 
jplatyodon, differs in the shape and smaller relative size of the teeth (PI. XXIV, figs. 6, 6'). 
They are more slender in proportion to their length than in Ich. communis (ib., figs, 5, 5'), 
and are straighter than in Ich. tenuirostris. Their base is cylindrical, less ventricose 
than in Ich. platyodon (ib., fig. 4'), and more finely and regularly fluted than in Ich. 
co7nmunis. A smooth boundary divides the base from the enamelled crown, and this is 
traversed by fine longitudinal grooves converging to the apex. The transverse section of 
its base is nearly circular; it tapers gradually to the apex, which is nearer the posterior 
line or contour than the axis of the tooth. 
The vertebral centrum has a greater proportional fore-and-aft extent than in Ich. 
^ ‘ Keport,’ nt supra, p. 116 . 
