1126 



CLASS REPTILIA. 



with young figured by Professor Seeley all belong to a single species. 

 This genus may be divided into two groups from the structure of the 

 pectoral limb. In the Longipinnale, or least specialised group, which is 

 unknown above the Lias, the orbit is generally of very large dimensions. 

 The pectoral limb (fig. 1027) is characterised by having the third digit, 

 or that arising from the intermedium, comprising only a single longi- 

 tudinal row of bones, and consequently having only one centrale ; while 

 there are usually only four longitudinal rows of phalangeals in this limb, 

 and the radius is nearly square, with a distinct notch in its anterior 

 border (fig. 1027). This group may be further subdivided into minor 

 groups. One of the most aberrant forms is the huge /. platyodon, of the 



Lower Lias, which attained a length 

 of nearly forty feet. This species is 

 characterised by the pelvic paddles 

 being nearly as large as the pectoral ; 

 but still more readily by the teeth 

 (fig. 1028), which have compressed, 

 carinated, and smooth crowns, and 

 the roots covered with a coating of 

 cement. /. lonchiodon is an allied 

 Lower Liassic form with cylindrical 

 and fluted teeth. Another type is 

 indicated by /. tennirostris and /. 

 latifrons (longirostris), in which the 

 cranial rostrum (fig. 1029) is greatly 

 elongated and very slender. The 

 paddles (fig. 1027) are remarkable 

 for the large size of the component 

 bones ; and in /. tenuirostris there is 

 a small circular vacuity between the 

 radius and ulna, indicating the last 

 remnant of the large vacuity found 

 in Mixosanrus. Lastly, /. acittiros- 

 frz's, I. zetlandicus, and /. integer 

 are three Upper Liassic members of 

 this group indicating a transition to 

 the next group ; the radius of the 

 latter species having no anterior notch. The skull of I. zetlandicus is 

 shown in fig. 1030. 



In the more specialised or Latipinnate group, which ranges from the 



Fig. 1028. — Lateral and profile views of a 

 tooth of Ichthyosaurus p/atyodon ; from the 

 Lower Lias of Dorsetshire. 



Fig. 1029. — Left lateral view of the skull of Ichthyosaurus latifrons ; from the Lower Lias 

 of Leicestershire. Reduced. 



Lias to the Chalk, the orbit is usually relatively smaller than in the last 

 group. The pectoral limb (fig. 1031) has the third digit, or that arising* 

 from the intermedium, with a double longitudinal row of bones, and there 

 are consequently two centralia. There are, moreover, never less than 



