ICHTHYOSATTRID^. 13 



Occasionally, as in the case of I. tenuirostris, mentioned on p. 85, 

 there are three distal facets on the proximal limb-bone ; but the 

 third is clearly formed by the upthrusting of the intermedium, and 

 is, therefore, if Baur's interpretation be correct, not homologous 

 with the third facet of Oplithalmosaurus. 



It has been suggested that this genus should be subdivided into 

 two or more genera ; and if this view be adopted, the two under- 

 mentioned groups should form distinct genera, although there are 

 signs of a transition from the one to the other. It would, however, 

 be difficult to know when to stop if generic divisions were made ; 

 and it therefore appears to the author to be more convenient, both 

 for the palaeontologist and geologist, to retain for the present the 

 genus in its original wide sense. 



A. Latipinnate Group 1 . 



Orbit of moderate size. Teeth with fluted, cylindrical crowns, 

 and their roots either invested with a smooth coat of cement, or 

 fluted in a coarser manner than the crowns. Pectoral limb (figs. 20, 

 22, 24) with the third digit, or that arising from the intermedium, 

 comprising two longitudinal rows of bones, and consequently two 

 centralia (fig. 24) ; not less than five longitudinal rows of phalan- 

 geals in this limb ; and the radius (fig. 8) transversely elongated 



Fig. 8. 



Ichthyosaurus intermedins. — Dorsal aspect of left radius; from the Lower Lias 

 of Street. £. u, facet for ulna ; i, do. for intermedium ; r', do. for 

 radiale. 



and its anterior border entire. Never any trace of a vacuity between 

 the radius and ulna. Pectoral limb much larger than pelvic (fig. 20). 

 Neural spines of dorsal vertebrae usually tall and narrow. 



This group comprises the more specialized forms, and includes 

 the type species. Its range in time extends from the Lias to the 

 Chalk. 



1 Kiprijanoff, Mem. Ac. Imp. St. Petersbourg, vol.xxviii. art. 8, p. 88(1881). 

 The term pinnate has been substituted for jrinnipedine in this and the next 

 group. 



