ICHTHYOSArRID-S. 41 



Humerus (fig. 17) and femur (fig. 18) without strongly developed 

 trochanteric ridge on dorsal surface. Cervical and anterior dorsal 

 vertebrae with subcylindrical centra ; and the superior costal arti- 

 culation of the former in great part or wholly on the centrum. 



This subgroup in the character of its humerus connects the 

 preceding one with the Acutirostrine subgroup of the Longipinnate 



Ichthyosaurus (cj:) intermedins. — Dorsal and distal a spects of the right femur ; 

 from the Lower Lias of Street. \. t, tibial, /, fibular facet. 



group. The cranial rostrum is never greatly elongated. The femur 

 (fig. 18) is always unsymmetrical, long and narrow, with only 

 a slight expansion of the distal extremity. In the humerus the 

 proximal extremity has a more or less oblong contour, with the 

 longer axis placed parallel to the longer axis of the distal surface. 

 The pubis and ischium are long and narrow. 



Ichthyosaurus communis, Conybeare l . 



Syn. Ichthyosaurus chiropolyostinus, Hawkins 2 . 



The type species. Teeth (fig. 19) relatively large, with short, 

 thick crown, and the flutings on both crown and root strongly 

 marked. Skull (fig. 21) with the length of the rostrum usually 

 equal to rather less than three times the longer diameter of orbit. 

 Supratemporal fossa small and triangular; and maxilla excluded 

 from the narial aperture by union of premaxilla with lachrymal. 

 Usually sixteen or seventeen plates in sclerotic. Vertebral centra 



1 Trans. Geol. Soc. ser. 2, vol. i. pt. i. p. 108 (1822). 



2 Memoirs on Ichthyosauri &o>, p], vii. (1834;. 



