MOLLUSCA. GASTEROPODA 239 



Pleurotomaria. Shell trochifbrm, conical, turbinate, or 

 nearly discoidal ; interior nacreous. Umbilicus present or absent. 

 Aperture sub-quadrate or oval, outer lip sharp, with a slit which, 

 as the shell grows, becomes filled up, leaving a band on the whorls, 

 towards which the lines of growth are directed obliquely backwards. 

 Operculum horny. Ordovician to present day ; common in Jurassic. 

 Ex. P. anglica, Lias ; P. ornata, Inferior Oolite. 



Murchisonia. Shell turreted, with many, more or less 

 angular whorls, provided with a band as in Pleurotomaria. Aperture 

 oblong, with a slit, and a very short anterior canal. Ordovician to 

 Trias ; mainly Devonian and Carboniferous. Ex. M. vemeuiliana, 

 Carboniferous Limestone. 



Bellerophon (fig. 99). Shell globular, usually with a narrow 

 umbilicus on each side ; whorls few, 

 embracing, symmetrically coiled in one 

 plane. Aperture sub-circular or oval, 

 with a deep median slit, which is re- 

 placed by a band or keel dividing the 

 shell into two similar parts ; columellar 

 edge often with callus. Ordovician to 

 Permian ; maximum in Carboniferous. ~pig. 99. Bellerophon, from 

 Ex. B. te?udfascia, Carboniferous Lime- the Carboniferous Lime- 



+ stone, showing the slit in 



the aperture, x % . 



Emarginula. Shell conical, sur- 

 face generally ornamented with a trellis-work of longitudinal and 

 transverse ribs ; apex not perforated, curved posteriorly. Anterior 

 border with a well-marked slit, which becomes filled up during 

 growth, leaving a raised band. Muscular impression horse-shoe 

 shaped ; no internal septum. Jurassic (perhaps Carboniferous) to 

 present day. Ex. E. Jissura, Coralline Crag to present day. 



Fissurella. Shell similar to Emarginula, but more or less 

 depressed ; apex perforated and nearer the anterior than the 

 posterior border ; no marginal slit. Muscular impression as in 

 Patella. Fissurella is divided into several sub-genera ; many of 

 the fossil species belong to the sub-genus Fissuridea. Jurassic to 

 present day. Ex. F. crassa, Recent ; F. grceca, Coralline Crag to 

 present day. 



