766 



DIVISIONS OF THE GASTROPODA. 



Fig. 649. — Pleuroto77iaria Agave. 

 (Billings.) 



Ordovician. 



Fig. 650. — Plenrotomaria Dryopc 

 (Billings.) 



Ordovician. 



among the Trochidee, the only genus of the family which still sur- 

 vives is Pleurotomaria itself. 



The genus Pleurotomaria is a " persistent type," the oldest forms 

 appearing in the Upper Cambrian, while four living species have 

 been detected. The genus is very largely represented in the Ordo- 

 vician, Silurian, Devonian, 

 and Carboniferous rocks. 

 A very limited number of 

 species has been hitherto 

 obtained from the Permian 

 deposits. In the Meso- 

 zoic rocks the genus again 

 shows a great develop- 

 ment, the Secondary forms 

 being commonly more or- 

 nate than those from older 

 rocks. Very few Tertiary 

 species are known, and the 

 few living species are found 

 in the Antilles and in the 

 Japanese seas. 



The form of the shell in 

 Pleurotomaria (figs. 649, 

 650) differs considerably in different types, being most generally 

 similar to that of Trochus or Turbo, but being in other cases very 

 much flattened out and depressed. The shell consists of a few 

 whorls, which are generally in close contact ; but the last whorl 

 may be disconnected from the others, or the shell may be entirely 

 evolute (Odontomaria). The aperture of the shell is subquadrate, 

 and the outer lip (fig. 648) exhibits a deeper or shallower slit. As 

 the shell grows, this slit becomes progressively filled up, forming a 

 well-marked band on the whorls. By this character Pleurotomaria 

 may generally be distinguished readily from such shells as Trochus 

 and Turbo. 



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 Leptomaria. In the Carboniferous Polytremaria the 

 band on the whorls is partially obliterated, and is perforated by a linear 

 series of minute foramina. In the Jurassic Ditremaria there is a kidney- 

 shaped aperture, consisting of two foramina united by a slit, in the band 

 near the outer lip. Lastly, in the Devonian Odontomaria the shell is 

 tubular and evolute. 



The Silurian genus Euomphalopterus is regarded by Lindstrom as an 

 extreme form of Pleurotomaria. In this type the shell is a depressed 

 spiral, the slit-band forming a wide alation, perforated by canals which 



