PLAGIOSTOMI. 129 



(fig. 42) has been found in the carboniferous formations near 

 Bristol and Armagh, and the genus is peculiar to that geolo- 

 gical period. 



A form of tooth more closely resembling the crushing-teeth 

 of Cestracion, is that on which the genus Acrodus is founded, 

 with species ranging from triassic strata to the upper chalk 

 of Maestricht. The one here selected (fig. 43) is the Acrodus, 

 nobilis, from the lias of Lyme Eegis. The upper figure shews 

 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 fossilists regarded these teeth 

 as petrified Vermes ; but the structure, 

 as shewn by the microscope, is closely 

 similar to that of the teeth of Cestra- Fig. 43. 



cion* Portions of the jaw of the A C- Acrodus noUlis (tooth). 



rodus have been discovered which shew 



that these teeth were arranged, as in Cestracion, in oblique 

 rows, with at least seven teeth in each row. Acrodus lateralis 

 is a muschelkalk fossil, A. hirudo a Wealden fossil, and A. 

 transversus a cretaceous fossil. No tooth referable to the 

 genus has been found in any tertiary stratum. 



The nearly allied Strophodus is represented by Str. angus- 

 tissimus in the muschelkalk, by several species in the oolites 

 (but not in lias), and by the Str. asper in the chalk of 

 Lewes. 



The genus Ptyclwdus is founded on teeth usually of large 

 size, and of a more or less square form (fig. 44). The crown 

 is deeper than the root, which is obtuse and truncate. The 

 enamelled summit of the crown is granulate at the margin, 

 and raised in the middle into an obtuse eminence, disposed in 

 large transverse, parallel, sometimes wavy and rather sharp 



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

 K 



