MKKIMAM: TKIASSIC TOITTIIYOSAl ' I,' I A. 107 



The jit(/<iJ is considerably expanded horizontally, and is comparatively thin 

 vertically, its transverse diameter equaling more than twice its vertical thick- 

 ness. Its vertical diameter below the middle of the orbit appears somewhat less 

 than in Baptttnotion, and less than in most forms of Ichthyosauri!*. The pos- 

 terior end reaches upward and backward to meet the quadratojugal above the 

 inferior temporal notch. 



The exposed portion of the post orbital appears to have an approximately 

 triangular form, though its upper posterior portion may extend backward 

 beneath the supratemporal as in Sliaslaxa/irns, and its anterior limits above the 

 orbit are not clearly determined. No suture line between the prcfroi/tal and 

 postfrontal can be distinguished on any specimen examined, nor is there a line 

 of demarcation observable between postfrontal and postorbital. The anterior 

 end of the prefrontal is overlapped by the lachrymals and the nasals. The 

 posterior end of the postfrontal extends back over the squamosal almost to the 

 posterior end of the temporal region. 



A well preserved sclerotic ring is shown in specimen 9913, and a complete, 

 unbroken, and apparently undistorted ring in no. 9954. In each specimen a 

 series of plates comprising one-half of the ring shows eight or nine plates, 

 making a total of sixteen to eighteen. Owen counted seventeen in Ichthyosau- 

 rus communis, and Fraas gives the number as varying from fifteen to twenty 

 in Ichthyosaurus. Gihnore finds but fourteen plates in Baptanodon, and An- 

 drews gives the same number for Opthalmosaurus. In specimen 9913 the inner 

 or median portion of each sclerotic plate bends sharply around the equator 

 of the eyeball and extends over the inner side of the eye showing a flattening 

 of the eyeball similar to that described for Ichthyosaurus by Owen 50 and for 

 I!j>tanodon by Gihnore. 51 The sclerotic ring is smaller compared with the size 

 of the skull than in any of the later ichthyosaurs. In specimen 9913 the great- 

 est diameter of the slightly distorted ring is a little more than 10 per cent, of 

 the total length of the skull. In no. 9954 the greatest diameter is less than 10 

 per cent, of the total length. In Ichthyosaurus platyodon and /. breviceps the 

 ring is relatively 40 or 50 per cent, longer. 



The lateral temporal bar of Cymbospondylus petrinns is always charac- 

 terized by uncommon length anteroposteriorly. Its height is somewhat less 

 than in most ichthyosaurs. Relative to the length of the cranial region it is 

 much less elevated than in Baptanodon. Behind the jugal the inferior side of 

 the temporal bar is sharply cut out so as to form with the slightly concave coro- 

 noid region of the mandible an elliptical opening with its long diameter 

 extended anteroposteriorly. According to the suggestion of McGregor" the 



5" Owen, E., Liassic Reptilia, part 3, p. 103. London, 1881. 

 si Gilmore, C. W., Am. J. Sc., vol. 23, p. 194, 1907. 

 - McGregor, J. H., Mem. Am. Mus., vol. 9, p. 91. 



