GASTEEOPODA. 



29 



Fig. 415. — Pleurotomaria Agave. Lower Silurian. 

 (Billings.) 



The genus Pleurotomaria comprises a great number of 

 Palasozoic univalves, which occur in the Silurian, Devonian, 

 and Carboniferous formations. In sediments later than the 

 Carboniferous the genus is largely represented, extending 

 even to the close of the Mesozoic period. In the Jurassic 

 period especially the genus has a great development, most of 

 the forms being more ornate than those from the older rocks. 

 In the Cretaceous rocks various species of the genus still 

 appear, but very few Tertiary forms are known, and only two 

 living species have been as yet detected. The form of the 

 shell in Pleurotomaria' (figs. 415, 416) differs considerably 

 in different cases. Very commonly the shell is very similar 

 to that of Trochus. In 

 other cases it more 

 nearly resembles Tur- 

 ho ; and sometimes it 

 is very much flattened 

 out and depressed. The 

 shell consists of few 

 whorls, of which the 

 last may be discon- 

 nected from the others, 

 and is essentially dis- 

 tinguished by its sub- 

 quadrate aperture, with 

 a deeper or shallower 

 slit in the outer lip. 

 As the shell grows, 



this slit becomes progressively filled up, forming a well- 

 marked band on the whorls. By this character Pleuroto- 

 maria may generally be distinguished readily from such 

 shells as Trochus and Tiirlo. 



Many subordinate types are included in the comprehensive 

 genus Pleurotomaria, in the wide sense. Thus a number of 

 Secondary types admit of separation from Pleurotomaria 

 proper by their possession of a very deep slit and a narrow 

 band, and these may be grouped together under the name of 

 Lerptomaria. In the Carboniferous Polytremaria the band on 

 the whorls is perforated by a linear series of minute foramina. 



Fio- HQi.—PUiirotoriiaria Dryopc. Lower Silurian. 

 (Billings.) 



