ICHTHYOSAURUS. 77 



lateral borders raised into prominent ridges in advance of the orbits ; 

 maxilla forming middle of inferior border of narial aperture. Neural 

 spines of dorsal vertebras of considerable height ; vertebral centra 

 long and moderately cupped. Pectoral paddle (comp. fig. 27) long, 

 with anterior border of radius, radiale, and adjacent carpale in second 

 transverse row notched; component bones relatively large but 



Fig. 28. 



Ichthyosaurus zetlandicus. — Superior and right lateral aspects of the skull ; 

 from the Upper Lias of Normandy. Reduced. Pmx, premaxilla ; Mx, 

 maxilla; N, nares; Net, nasal; Fr, frontal; Prf, prefrontal; Ptf, post- 

 frontal; Pa, parietal; J, jugal; QuJ, quadratoj ugal ; Sq, squamosal; 

 St, supratemporal ; Por, postorbital ; A, orbit ; S, supratemporal fossa ; 

 Scl, sclerotic ; Md, mandible ; d, dentary ; op, splenial ; ang, angular ; 

 k, articular. {After Zittel.) 



markedly polygonal, and the first longitudinal row of ulnar phalan- 

 geals distinctly oblong, with their longer diameter transverse. A 

 small fifth row of phalangeals probably present in some cases. 

 Coracoid (fig. 30) elongated antero-posteriorly ; scapula of the 

 general type of that of /. platyodon (infra, p. 101). 



The radius and ulna have no interval between them, and the 

 former articulates by a large surface with the intermedium. 



The only characters by which this form can be distinguished from 

 /. acutirostris are the form of the rostrum and supratemporal fossa 4 . 

 Although slight differences in the relations of the bones of the 

 quadratic region are found in the crania described as /. longifrons, 

 yet these forms are evidently so closely related that they may well 

 be classed under one specific name. The pectoral limb of the so- 

 called /. longifrons presents all the characters of that oil. acutirostris. 

 If the form commonly described as /. quadriscissus belong to this 

 rather than to the preceding species, that name has the priority, but 

 its dubious authenticity (p. 73, note) is a bar to its adoption. 



