XIII. 



ICHTHYOSAURUS. 



335 



tail, which is reached (probably) at about the looth joint. Guided 

 by the general view thus acquired, we may now consider the 

 characteristics of species, which are almost wholly to be founded 

 on the vertebrae. 



ICHTHYOSAURUS TRIGONUS. Owen. 



The Kimmeridge clay of Shotover and Swindon are the principal 

 localities for this species, which is frequently met with, and of 

 various sizes. The largest may have reached 20 feet in length, 

 with a cervical 3-5 inches broad, posterior dorsal vertebra 5-25 

 inches ; the smallest may have been 5 feet long. 



Occipital region. The basi-occipital is a hemispherical mass, with 

 the usual lateral and inferior expansions, and a small groove 

 above. 



Cervical region. Three examples occur of the anchylosed first 

 and second vertebra. One measures 1*00 inch across, another 2*90, 

 a third 3*50 (including the apophyses). The anterior face is ex- 



Diagram CXXVI. Ichthyosaurus trigonus. Scale one-fifth of nature. 

 i. Cervical vertebra (fourth), seen in front. 2. The same, seen on the left side. 



3. The same, seen from above. 



cavated^to fit the basi-occipital, the edge revolute. The outline 

 of the faces is rather pentagonal than trigonal ; the lower side 

 slopes converging to form an angle exceeding 90 in the young, 

 but falling short of that arc in the old. The neural canal ends in 

 an arched outline against the cranial bone, and is gently concave, 

 with several small foramina. In the young and middle-aged speci- 

 mens the supplementary inferior bone can be traced, by suture, 

 but not in the aged individual. In the young example each portion 

 of the vertebra bears a neuro-spinal cicatrix ; in the older specimens 

 the anterior one fails or grows indistinct. The di- and par- 

 apophysial bases are somewhat irregularly placed. 



