Merriam. j 



Triassic Ichthyopterygia . 



75 



a posterior facet, each extending to the intervertebral articular 

 surface of the centrum. Although somewhat broken, the anterior 

 facet appears to be larger than the posterior. On the second verte- 

 bra the inferior apophysis is situated on the anterior half of the 

 centrum and possesses a large anterior facet similar in form but 

 smaller than that on the rear of the vertebra iu front of it. 

 The posterior border of the second cervical is slightly elevated, 

 as is also the anterior border of the third, and a small ossicle 

 may have been situated between them. A small bone found with 

 the anterior cervicals has the form and dimensions required for 

 the intercentrum in front of the first vertebra and probably 

 represents that element. 



The facets for the reception of the intereentra on the first 

 cervical in this series are much like those figured by Owen* for the 

 second vertebra of Ichthyosaurus longifrons. It is, however, very 

 doubtful whether the most anterior vertebra here is either atlas 

 or axis, as the anterior end of the centrum presents a deep 

 concavity similar to that in all the subsequent vertebra?, and 

 entirely unlike either the anterior or posterior face of the atlas 

 or the anterior face of the axis in Ichthyosaurus, as described by 

 Owen. On the other hand, the small size of the vertebra, as 

 compared with those behind it, and the rapid decrease in the size 

 of the intereentra posteriorly indicate that the first centrum is 

 probably not far removed from the skull. In alexandrm there 

 is evidence of the existence of intereentra , at least as far back 

 as the fourth cervical. It is pretty clear that there were more 

 cervical intereentra in Shastasaurus than in Ichthyosaurus, as the 

 last intercentrum in Ichthyosaurus is between the axis and the 

 third vertebra. 



Through a large part of the caudal region hcemapophyses are 

 well developed. Unlike the lower arches of Ichthyosaurus, their 

 inferior extremities unite to form long spines, or in other words, 

 produce Y-shaped chevron bones (PI. 5, fig. 1, and PI. 7, 

 fig. 3). In perrini the spinous portion of the arch may be 

 double the length of the pedicels. The articulation of the lower 

 arches is intervertebral, and pairs of small facets are developed 

 on the adjoining posterior and anterior margins of the centra 

 with which they are in contact. 



* Liassic Eeptilia. PL 23, fig. 5. 



