ioS 



HARDWICKE'S SCIENCE-GOSSIP. 



dorsal fin-spines. More recent discoveries in America 

 have proved the identification of the latter to be 

 incorrect, and Prof. Cope has shown that the 

 remains in question are the fin-rays of Teleostean 

 fishes, which he places in a family under the name of 

 Pelecopteridoe. 



specific characters and arrangement in the mouth. 

 Species may generally be founded, with a considerable 

 approach to accuracy, upon detached teeth, from a 

 consideration of the ornament of the crushing 

 surface ; this has been proved by the discovery of 

 numerous large groups (each evidently the remains 



Fig- 73-— Tooth of Ac rod us nobiVis. 



Fig. 74.— Ornamentation of tooth of Acrodus- 

 nobilis. 



II,;. 71. — Jaw of recent Cestracion Philippi 



Fig. 75. — Transverse 



section of spine of 



I/ybodus. 



Fi». 76. — Transverse 



section of spine of 



Acrodus. 



Fig. 77. — Tooth of 

 Acrodus minimus. 



I '3- 79- — Tooth of S/rop/iodus 

 reticulatus. 



Fi ? . 78.— Tooth of 

 Sf> ofhodiis Javosus- 



Fig. So. — Tooth of 

 Stropkodus asfer. 



Fig. 72.— Jaw olStrophodus medius. (After Owen.) 





Fig. Sr. — Tooth of Ptychodtts mammillaris. 



As the teeth of Ptychodtts are so well known to 

 all acquainted with the fossils of the Chalk, it is 

 unnecessary to describe their general shape in detail 

 here, and reference need only be made to their 



of a single mouth), in which all the forms may be 

 easily recognised as modifications of a single type. 

 About ten species occur in the English Chalk, and of 

 these the commonest are P. dccurrens, P. polygyms, 



