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 fin- 

 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 narrow 

 paddles of Ichthyosaurus platyodon. 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 Rooms at Burlington House — 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 semicostatus, characteristic of the greyish 

 limestone (Lower Lias) of Lyme Regis. 



e. Ichthyosaurus lonchiodon, 1 Ow. PL 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. 

 jolatyodon, 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. 

 communis. 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. 



1 'Report,' ut supra, p. 116. 



