110 FOSSIL REPTILIA OF THE 



The skull with the right side shown in profile (PI. XXIX, 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 7^ 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 Ichthyosaurus breviceps is thus shown to have approached in size is 

 Ich. platyodon, Conybeare ; but, like Ich. trigonodon, Theod., 1 it had fewer and propor- 

 tionately larger teeth. 



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

 Ich. platyodon, 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. platyodon the number of teeth 

 is thirty -two ; in that of Ich. breviceps 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. breviceps, is three times and two thirds 

 that of the long diameter of the orbit ; in Ich. platyodon 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, pari passu, with that of the general bulk of the animal. 

 The sclerotic plates, thirteen in number, have been compressed from within at the side 

 opposite to that exposed, and the parts which were abruptly bent upon the midpart of the 

 eyeball have been pushed into line, and fractured at the bend with the fore parts of those 

 plates. 



The mandible shows a specific variety in the proportions of its constituent elements. 

 The angular (PI. XXIX, fig. 1, 3l), which in Ich. platyodon has less depth, and in Ich. 

 communis much less depth than the surangular, opposite the back part of the orbit, has in 

 Ich. breviceps greater depth, and it extends further forwards, viz. within nearly one fourth 

 of the fore end of the ramus, instead of terminating within one third {Ich. platyodon), 

 or before it reaches half way to that end {Ich. communis). 



The maxillary is excluded from the external nostril by the junction of the pre- 

 maxillary (22) with the lacrymal (73). The malar (26) extends further forward in a 

 slender pointed form, in advance of the orbit, between the lacrymal and maxillary (21). 

 The hinder expanded sutural border of the nasal (15) is sculptured by some strongly- 

 marked ridges and grooves. 



The swollen base of the tooth is impressed by longitudinal grooves, fewer in die 

 upper (ib., fig. 3) than in the lower ones (fig. 4); the enamelled crown shows finer longi- 

 tudinal lineations, and the fore border is slightly trenchant. The crown is relatively longer 

 and more slender than in Ich. trigonodon, and less compressed than in Ich. platyodon. The 



1 And. Wagner, ' Beitrage zur Arten von Ichthyosaurus,' 4to, 1851, p. 34, tab. xvi, figs. 3 — G. 



