MKIMUA.M: TRIASSIC ICIIT1I YOSAUKIA. 



35 



dorsals are each marked by a well-developed ridge (fig. 30). This lateral rib 

 is situated in the middle of the lateral face of the spine, and possibly represents 

 the original column of the spine from which the anteroposterior expansion has 

 developed. 



In ti/tfix/nxfiiiriix <t//i*/>iinis and S. c/nri/i the spines show little transverse 

 flattening in the dorsal region and sometimes none at all; but may be consid- 

 erably thickened and perfectly circular in cross-section distally. In the type 

 specimen of S. <i1lixi>innx the middle dorsal spines seem to have thin, blade-like 

 anterior and posterior expansions of no great width, for a short distance above 

 the zygapophyses (pi. 18, figs. 2a and 2&). In a specimen of S. careyi (pi. 18, 



34a 



34b 



Ki};s. 34-34c. Mixosaurm (?) atavus (Quenstedt). Neural arch of an anterior dorsal vertebra, 

 natural size. (After Fraas.) Fig. 34<i, anterior view; 346, posterior view; 34c, lateral view. 



figs. 5(1 to 5r) the distal end of the spine is considerably thicker than the 

 middle section, and shows no anterior or posterior blade-like expansions, but is 

 circular in cross-section. In S. altispinus the terminal portion of the spine is 

 hollowed out by an anteroposterior groove. In S. careyi it is evenly convex or 

 domed. 



In view of the fact that thickening of the neural spines is the rule among the 

 Triassic Ichthyosauria, and is most persistent in the anterior portion of the 

 vertebral column where the development of a sculling tail would have the least 

 influence on the character of the neural spines, it is probable that the arches 

 of the ancestral forms were of the round or thickened rather than of a later- 

 ally compressed type. The rounded type is occasionally found in aquatic forms, 

 but is most common among running types. 



In the cervical, dorsal, and anterior caudal regions zygapophysial facets 

 are developed on the upper arches of all Triassic ichthyosaurs. As was first 



