MERRIAM: TRIASSIC ICHTHTOSAUBIA. 



119 



eriorly. They are not divided along the median line, as in some ichthyosaurs. 

 The anterior end or the point of the V of each median piece extends forward on 

 the median line as an overlapping knob-like process. Between the lateral ends 

 of the median pieces are inserted the ends of the heavy discrete lateral pieces. 



The plastron presents as a whole an appearance of somewhat greater 

 strength and robustness than that of the later ichthyosaurs. 



Pectoral Arch. The entire pectoral arch is well known excepting the inter- 

 clavicle, which has never been recognized (pi. 11). 



The xc<(/>i(l<t is not like that of any other ichthyosanr. The body of this 

 element is strongly recurved so that the much expanded distal end is turned 

 almost at a right angle to the proximal portion. The anterior part is also 

 much broadened just in front of the proximal end. The form of this element 

 is quite unlike that in any other Triassic genus. It approaches most closely 

 the general form in Ichtli yosatirtiN, particularly in the character of the ante- 

 rior expansion, which occupies the position of the anterior hook in Shasta- 

 NftHrH*. It differs from the type seen in Ichthyosaurus in its much greater 

 proximal and distal expansion. 



The form of the sickle-shaped coracoid does not 

 correspond to that in any other ichthyosaurian genus. 

 The nearest approach is found in Delphinosaurus of 

 the Calif ornian Upper Trias. In Delphinoswrus 

 the position seems to be reversed anteroposteriorly, 

 and the large perforation just anterior to the proxi- 

 mal articulation is absent. Perforations and emargi- 

 nations in this region of the coracoid are ordinarily 

 not remarkably persistent features. This character 

 has however been recognized in all specimens of C. 

 Itch-inns thus far examined, and together with other 

 differences would seem to distinguish this form suf- 



ticielltlv 



The clavicle is uncommonly large and heavy. The 



middle portion is considerably expanded, but the ends are narrow. . The median 

 end seems to show a face of articulation for an interclavicle, but that element 



is unknown. 



Pectoral Limb. As yet we know certainly only the first three segments of 

 the pectoral limb in C. petrinus, no specimen having been found in which the 

 phalanges are in place or even certainly recognizable. Owing to early decom- 

 position of the thinner portions of the paddles they have probably been scat- 

 tered before the slow accumulation of calcareous mud could cover them. In all 

 of the Cymlospondylus specimens that have been examined the limb elements 

 are almost exactly similar to the limb of specimen 9154 previously described 

 (Merriam, 1905, 2). 



anterior perforation, 



