390 REMAINS OF AN ICHTHYOSAURUS. 



It presents the ichthyic character of the concavity of the arti- 

 cular surface on both the front and back part of the centrum 

 c ; with the character of coexisting diapophyses d and parapo- 

 physes p, not known in fishes, but which the Enaliosauria 

 present in their anterior trunk-vertebrae, in common with the 

 Dinosauria, Crocodilia, and other highly organized reptiles. 

 The generic characters of the Ichthyosaurus are manifested in 

 the shortness (i. e. the relatively small fore and aft diameter) 

 of the centrum as compared with its breadth and height, and 

 in the shape of the neurapophysial surfaces n p, and their 

 proportions to the neural surface n. With regard to the 

 specific character of this vertebral centrum, its proportions 

 pretty closely accord with those of the Ichthyosaurus acutus 

 from the Whitby Has; but this would be quite inadequate 

 ground for a reference of the Arctic Ichthyosaur to that spe- 

 cies in the absence of any evidence of the shape of its skull 

 and dentition. 



Figures 4 to 7 are of a terminal caudal vertebra, of the na- 

 tural size, apparently of the same species of Ichthyosaur and 

 probably from the same individual as the vertebrae figs. 1-3, 

 from the more advanced part of the body. 



The small caudal vertebra equally manifests the Ichthyo- 

 saurian characters in its degree of biconcavity and in the form 

 of the neurapophysial pits np; the lateral compression of 

 the centrum indicates the vertical development of the tegu- 

 mentary tail-fin it helped to support : on the under surface 

 are four surfaces for the haemal arches, which are articulated, 

 as in the Crocodiles, at the vertebral interspaces to two conti- 

 guous centrums. 



Pigs. 8 to 11 are portions of ribs. The long, free, thoracic- 

 abdominal pleurapophyses, or vertebral ribs, of the Ichthyo- 

 saurus are peculiar for the deep longitudinal groove which 

 impresses them on each side, giving to their transverse section 

 the form represented in fig. 10. Two fragments of ribs, figs. 

 8 and 9, found associated with the before-described vertebrae, 

 present this grooved character, and, with the vertebrae, afford 

 cumulative proof of the Ichthyosaurian nature of the Arctic 

 fossils represented in Plate XXXI. 



