LIASSIC ICHTHYOSAURS. 67 



case of Cetacea, showing minor modifications than do the toothed Icldhijopteri/gia, other 

 generic terms for some of these species may be proposed. 



C. Species. 



a. Ichthyosaurus breviceps, Ow., Tab. XXV. 



In the skeleton of this species (Tab. XXV, fig. 2) the skull is almost equally divided 

 between the antorbital part and that behind ; it is about one sixth the length of the 

 entire body, as represented by the vertebral column. This includes, in the specimen 

 figured, 125 vertebrae, of vphich 46 lie between the skull and pelvis. The neural spines 

 of such trunk-region are lofty, equalling along its major part the vertical diameter of 

 the rest of the vertebrae taken from the base of the spine. The intervals between the 

 spines are very narrow. The centrums are largest at and near the pelvic region. The 

 fore fin has five normal digital series, with smaller supplementary ossicles along both 

 fore and hind borders ; it is twice as long and as broad as the hind one. 



The specific characters are more fully exemplified in specimens of the skull of larger 

 individuals, which show that the proportions of the rostrum to the rest of the skull in 

 the smaller skeleton may be due to nonage, but the cranial conformation is the same. 



The skull with the right side shown in profile (Tab. XXV, fig. 1) was discovered in 

 the thick Liassic Limestone, called " Broad Ledge," at Lyme Regis ; and, as usual with 

 such fossils from this locality, is somewhat compressed. 



The length of the mandible is 3 feet 1\ inches ; that of the upper jaw from the fore 

 part of the orbit is barely 2 feet ; and from the fore part of the nostril is 1 foot 4^ inches. 

 These proportions indicate the character of the skull which suggested the specific 

 name. 



The species which Icldhyosaurus hreviceps is thus shown to have approached in size is 

 Ich. platyodon, Conybeare; but, like Icli. triffonodon, Theod.,^ it had fewer and propor- 

 tionately larger teeth. 



In the portion of the iqiper jaw in advance of the nostril of a well-preserved skull of 

 Ich. pJatijodon, the number of teeth is thirty ; whilst in Ich. breviceps they do not exceed 

 eighteen. In a corresponding extent of the lower jaw of Ich. j^latyodon the number of teeth 

 is thirty-two ; in that of Ich. hreviceps it is twenty-two. The length of the skull from the back 

 of the orbit to the fore end of the upper jaw, in Ich. hreviceps, is three times and two thirds 

 that of the long diameter of the orbit ; in Ich. plafyodon the length of the skull from the back 

 of the orbit forwards is four times and one third that of the orbit in one specimen, and four 

 times and a half that of the orbit in a larger specimen ; the size of the eye and of its bony 

 cavity not augmenting, apparently, joon'joasstf, with that of the general bulk of the animal. 



1 And. Wagnee, ' Beitrage zur Arten von Icbtbyosaurus,' 4to, 1851, p. 34, tab. xvi, figs. 3 — 6. 



