PLAGIOSTOMI 



109 



Fig 29. 



Cochliodus contortus, Ag. 

 (Carboniferous.) 



welded into a single dental mass in Cochliodus, the proportions 

 and direction of the rows being closely analogous. Whether 

 in Cochliodus, there were any small anterior prehensile teeth, 

 is hypothetical ; the large crushing dental plates must have 

 been admirably adapted to crack and 

 bruise the shells of mollusks and crus- 

 taceans. The Cochliodus contortus 

 (Ag.) (fig. 29) has been found in the 

 carboniferous formations near Bristol 

 and Armagh, and the genus is peculiar 

 to that geological period. 



Teeth referable to the genus Hy- 

 bodus occur in all the secondary rocks from the trias to the 

 chalk inclusive. 



A form of tooth which more closely resembles the crush- 

 ing-teeth of Cestracion, is that on which the genus A crodu-s is 

 founded, and which also ranges from , ,. v _. ...,._--<- ;<>- 



triassic strata to the upper chalk of 

 Maestricht. The species here selected 

 (fig. 30) is the Acrodus nobilis, from 

 the has of Lyme Eegis. The upper 

 figure shows the grinding surface, 

 which, from its finely and transversely 

 striated character and dark colour, has 

 suggested to the quarrymen the name 

 of " fossil leeches." The older fos- 

 silists regarded these teeth as petrified Vermes ; but the struc- 

 ture, as shown by the microscope, is closely similar to that of 

 the teeth of Cestracion * Portions of the jaw of the Acrodus 

 have been discovered which show that these teeth were ar- 

 ranged, as in Cestracion, in oblique rows, with at least seven 

 teeth in each row. Acrodus lateralis is a muschelkalk fossil, 

 A. hirudo a Weal den fossil, and A. transversus a cretaceous 



* See Owen's Odontography, vol. i., p. 54, pis. 14 and 15. 



Fig. 30. 



Acrodus nobilis (tooth). 

 (Lias.) 



