MERRIAM: TINASSIC ICIITIIYOSAriM A. 



In the anterior dorsal region the rib heads articulate mainly upon the large 



diapophyses, but the upper portion seems to have been distinctly in contact 

 with the base of the neural arch. (See tig. 26, p. 32.) About as far hack as 

 the twenty-fifth vertebra the bases of the upper arches are produced laterally 

 over the tops of the centra so that the lateral extremities are immediately over 

 the lateral faces of the diapophyses. The ends of these lateral spines of the 

 neural arches appear distinctly truncated, and the lateral faces were evidently 

 continuous with the articulating faces of the diapophyses. 



The rib articulation remains high up on the side of the centrum as far back 

 as the middle of the dorsal region, where it suddenly drops to the middle of 

 the centrum. 



In all specimens examined the lower arches of the caudals in this species 

 are very long, while the right and left elements are fused inferiority to form a 

 spine which may be considerably longer than the V-shaped portion above. The 

 articulation of the lower arches is intervertebral, and pairs of distinct facets 

 are formed on the adjoining margins of the centra with which the arches are 

 in contact. 



The long dorsal ribs of D. perrini are of the same general form as those in 

 SlidstdHdttrus, but do not show as pronounced outward curvature of the upper 

 portion of the articular face as in Shastasaurus altispinus. In the dorsal re- 

 gion the rib shaft is furrowed anteriorly and posteriorly for a large part of 

 its length. 



The pectoral arch was represented in the type specimen by a fragment of 

 a coracoid. In specimen 9082 (pi. 14) the scapula, 

 coracoid and probably the clavicle are preserved. 

 The scapula (fig. 143) differs from that of all other 

 ichthyosaurians in having a broadly sickle-shaped or 

 lunate form without an anterior hook as in most other 

 Triassic forms. The posterior margin is much 

 shorter than the anterior and is strongly concave. It 

 is separated from the proximal and distal margins 

 by sharp angles. The anterior margin is regularly 

 convex, and curves into the proximal and distal mar- 

 gins, from which it is not distinctly separated. 



The coracoid has the form of an ellipse with a Fig. 143. 



., , , , . Merriam. Scapula, X 



strong emargination at one end. The general outline 



is somewhat similar to that of Ichthyosaurus in (/ens Theodori. 



Large curved elements which may represent the clavicles are present on the 

 slab in specimen 9082, and are in contact with the scapula. No interclavicle is 

 known. 



With specimen 9082 both anterior Kmls are present with the pectoral girdle 



pmi 



