MOLLUSCA. GASTEROPODA 241 



protuberance or a fold. Operculum horny, multispiral, nucleus 

 central. Trias to present day. Ex. T. niloticus, Recent. There 

 are numerous sub-genera. 



Nerita. Shell thick, solid, ovoid or semi-globose, without an 

 umbilicus ; interior not nacreous. Spire very short. Surface smooth 

 or with spiral ribs. Aperture semicircular, entire ; outer lip thick, 

 the interior generally denticulate ; inner lip flattened, with callus, 

 and a straight denticulate border. Operculum calcareous, nucleus 

 excentric. Cretaceous to present day. Ex. X. ust v lata, Recent ; 

 N. globosa, London Clay and Bracklesham Beds. 



Neritina. Form similar to Xerita. Shell relatively thin, 

 usually smooth and with colour marking. Outer lip sharp, not 

 thickened, with interior not denticulate ; inner lip flattened, with 

 sharp or finely denticulate border. Eocene (perhaps earlier) to 

 present day. Lives in brackish or fresh water. Ex. X. aperta, 

 Headon Beds ; X. zebra, Recent. 



Macrochilina ( = 3facrocheilus). Shell elongate-oval, with 

 sharp spire, and last whorl high. Surface smooth or with growth- 

 line. No umbilicus. Aperture ovate, angular behind, sometimes 

 with a shallow anterior canal ; outer lip thin, inner lip with a weak 

 anterior fold. Silurian to Trias. Ex. M. arculata, Devonian. 



Loxonema. Shell turreted, spire very long ; whorls convex, 

 ornamented with sinuous growth-lines ; sutures deep. No umbilicus. 

 Aperture long, enlarging in front, with shallow canal ; outer lip 

 sharp, sinuous. Silurian to Trias (chiefly Carboniferous). Ex. 

 L. constrictum, Carboniferous. 



Pseudomelania. Shell elongate, with many nearly flat 

 whorls, without umbilicus, spire long, surface smooth or with 

 growth-lines. Aperture oval, entire, rounded in front, narrowed 

 and angular behind ; outer lip sharp. Columella smooth. Trias 

 to Eocene ; common in Jurassic. Ex. P. heddingtonensis, Corallian 



Scala ( = Scalaria). Shell turreted, spire elongate; whorls 

 numerous, very convex, sometimes separated, ornamented with 

 strong transverse ribs or sometimes with lamellae, frequently with 

 spiral ribs also. Umbilicus more or less distinct. Aperture circular, 

 entire, margin thickened. Operculum horny, paucispiral. Trias to 

 present day. Ex. S. scalaris, Recent ; >$'. groenlandica, Red Crag to 

 present day. 



w. p. 16 



