LIASSIC ICHTHYOSAURS. 75 



paddles of IchlJii/osaurus jilafi/odon. The least incomplete skeleton of this huge species 

 in the British Museum, the subject of Tab. XXVII, 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 

 Tab. XXVII, fig. 1, are of the species Arietites semicostatiis, characteristic of the greyish 

 limestone (Lower Lias) of Lyme Regis. 



e. Ichthyosaurus lonchiodon/ Oto. Tab. XX, figs. 6, 0'; Tab. XXVII, figs. 4 — 7. 



This species, which appears to have attained a bulk second only to that of the Ich. 

 platyodon, differs in the shape and smaller relative size of the teeth (Tab. XX, 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. plati/odo7i (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. 

 platyodon ; the neural arch and spine have a less vertical, in proportion to the fore-and-aft, 

 extent (Tab. XXVII, fig. 6) than in Ich. communis or in Ich. tenuirostris. Forty-five of 

 these vertebrae may be counted between the occiput and the pelvis; and as many 

 beyond may be made out in the subject of fig. 4, Tab. XXVII, as brings the number up 

 to 120, but the tail is incomplete. 



The length of the rostrum anterior to the orbit (Tab. XXVII, fig. 5) includes little 

 more than three longitudinal diameters of that cavity ; it is thus relatively shorter than 

 in Ich. platyodon, and it is relatively more slender than in Icli. intermedius. The mandible, 

 however, does not partake of this proportion, but is nearly as deep and strong, rela- 

 tively, as in Ich. platyodon (ib., fig. 2). The surangular (30) extends farther forward 

 than the angular (31) ; both disappear in the usual pointed form. The nostril is divided 

 from the maxillary by union of the premaxillary with the lacrymal. The pair of hyoid 

 elements (cerato-hyals) are preserved in situ in the subject of fig. 5 ; they are cylindrical, 



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



