LIASSIC FORMATIONS. 115 



The present species has afforded the chief ground of the restoration outlined in fig. 1 

 PI. XXXIIJ. The humerus is marked 53 ; the radius 54 ; the ulna 55. The fore 

 border of the radius is entire, not notched. The antibrachium supports, as usual, three 

 ossicles (56), followed by four rather smaller (5G')- Regarding these as carpals, they 

 support four metacarpal ossicles (57—57'), from which are continued five longitudinal 

 series of progressively diminishing phalanges. The anterior or radial metacarpal (57) 

 supports two digits, of which the radial may be symbolised as I, the next series as 

 digit II. The three other series, III, IV, V, are supported by their respective meta- 

 carpals. Each digit consists of numerous phalanges, progressively decreasing to the end. 

 A series of small supplementary ossicles is applied to the radial border of digit I, and to 

 the ulnar border of the digit V, so that seven ossicles may be counted in the same 

 transverse line along the middle third or fourth of the series. The number of phalanges 

 is, however, less than in Ich. communis, and the fin is relatively narrower. The characters 

 of the fore paddle above defined are well shown in specimens from the Lias of Lyme 

 Regis and of Street, in the British Museum. 



In the pelvic fin the femuris longer in proportion to its breadth, and the distal 

 expansion is relatively greater than in the humerus. The tibial ossicle of the three 

 tarsals has an emarginate tibial border ; the corresponding ossicle or phalanx of the 

 second and third series shows the same character. 



d. Ichthyosaurus platyodon, Cb. Plate XXIV, figs. 4, 4'; PI. XXXI, figs. 1, 2, 3, 



The skull of Ichthyosaurus platyodon (PI. XXXI, fig. 2), is somewhat longer in 

 proportion to the trunk than in Ich. communis. Taking as the trunk the extent of the 

 vertebral column to the pelvic arch, such extent includes, in the subject of fig. 1, one 

 length and a half of the skull, while in Ich. communis (PL XXVIII, fig. 1) it includes 

 rather less than two lengths, and, in Ich. intermedins (PL XXX, fig. 1) rather more. 



The skull of Ich. platyodon is longer in proportion to its breadth than in Ich. inter- 

 medius. The jaws are stronger from the greater relative depth of the mandible and 

 the less gradual attenuation to the rostral extremity. The orbit is of a full elliptic 

 shape, with less approach to the circular, than in Ich. breviceps (PI. XXIX, fig. ]). It 

 is relatively less than in the long- and slender-snouted kinds. The length of the rostrum 

 anterior to the orbit is three and a half times the longitudinal diameter of that cavity. 



The osseous circumpupillary ring includes thirteen sclerotic plates in the subject of 

 PL XXXI, fig. 2. The surangular (PL XXXI, fig. 2, 30) disappears between the 

 dentary (33) and angular (31). This element similarly disappears in a pointed form 

 between the dentary (33) and splenial (32), beyond the midlength of the ramus. 

 10 



