BRITISH FOSSIL REPTILES. 103 



especially characterized by a small triangular tubercle developed 

 at each of the four angles of the quadrilateral space which 

 forms the inferior surface, the tubercles being largest at the 

 anterior angles : this tubercle, with the corresponding one on 

 the adjoining vertebra, forms the surface of articulation for the 

 caudal haemapophyses, which are short and slightly curved 

 simple bones, not joined together at their distal extremities so 

 as to constitute a bone of the chevron shape, as in the existing 

 Saurians : these haemapophyses disappear in the last 20 vertebrae. 



Besides their double-cupped articular surfaces and the pre- 

 sence of the costal tubercle, the vertebral centres of the Ichthyo- 

 sauri generally differ from those of the Plesiosauri in having a 

 more angular contour, which sometimes forms nearly a true 

 hexahedron. 



Each sterno-costal arc consists, as Mr. Hawkins correctly 

 states*, of five bones. These are slender transversely elongated 

 ossicles, which overlap each other in the way which has been 

 described in the Plesiosaitrus. The median piece is generally 

 symmetrical in shape, and sends off from its middle part a short 

 thick process both forwards and backwards, or in tiie longitu- 

 dinal axis of the body. In the symmetrical median pieces, the 

 elongated lateral processes are continued from the middle of 

 the short longitudinal one ; in the unsymmetrical median pieces 

 one lateral process comes off from the anterior and the other 

 from the posterior portion of the longitudinal process. The 

 lateral processes bend slightly backwards, and diminish to a 

 point. A slender cylindrical styliform process pointed at both 

 extremities is spliced as it were to the anterior part of each 

 lateral., process of the median piece, which it equals in length ; 

 it extends however a short distance beyond its lateral extremity. 

 A second styliform ossicle is similarly adapted to the anterior 

 part of the lateral or outer extremity of the preceding piece, 

 but the lateral extremity of the third process is not pointed 

 but is slightly expanded and abruptly truncated in order to join 

 the lower extremity of the true ribf. In the singleness of the 

 median piece and the development from its middle part of a 

 short longitudinal process may be discerned an affinity to cer- 

 tain Lacertian Sauria, as the Polychrus marmoratus, Cuv., 

 while in the double overlapping lateral pieces we perceive a re- 

 semblance to the condition of the abdominal ribs in the Croco- 

 dile. If the median piece in the Ichthyosaurus were removed, 



* Memoirs on Ichthyosauri, p. 21. 



t In a rare and beautiful specimen of this sterno-costal apparatus of a spe- 

 cies of Ichthyosaurus from the lias of Lyme Regis, kindly transmitted to nie 

 for examination by Mr. Conybearc. there are 19 slender arcs^ each composed 

 of the five pieces above described. 



