240 MOLLUSCA. GASTEROPODA 



Euomphalus. Shell depressed, discoidal or conical, with 

 a wide and large umbilicus ; whorls convex with a ridge on the 

 upper surface. Aperture polygonal ; outer lip with a slit on its 

 upper surface. Silurian to Trias ; maximum in Carboniferous. 

 Ex. E. pentangulatus, Carboniferous Limestone. 



Omphalotrochus ( = Horiostoma of some authors). Form 

 similar to Euomphalus. Whorls ornamented with spiral keels and 

 numerous transverse stria? or fine ribs. Aperture without a slit. 

 Ordovician to Carboniferous. Ex. 0. discus, Silurian. 



Turbo. Shell solid, turbinate or conical, whorls convex, in- 

 terior nacreous. Aperture large, circular, entire, slightly produced 

 anteriorly ; outer lip sharp. Columella curved, flattened. Im- 

 perforate, or with a small umbilicus. Operculum thick, calcareous, 

 exterior convex, interior flat and spiral, nucleus central or sub- 

 central. Jurassic (perhaps also Palaeozoic) to present day. Ex. T. 

 marmoratics, Recent. There are numerous sub-genera. 



Phasianella. Shell elongated, oval or oblong, smooth, polished, 

 without an umbilicus, interior not nacreous. Aperture oval, entire, 

 rounded anteriorly, angular posteriorly ; outer lip thin, simple, 

 sharp. Columella smooth, flattened. Operculum calcareous, with 

 an excentric nucleus. Upper Cretaceous (or earlier) to present day. 

 Ex. P. australis, Recent ; P. gosauica. Upper Cretaceous. 



Amberleya. Shell turbinate, elongate, without umbilicus. 

 Whorls ornamented with several spiral keels which are usually 

 spiny or nodular ; between the keels are numerous transverse strise 

 or fine ribs. Base rounded. Aperture sub-oval ; outer lip often 

 crenulated. Trias to Cretaceous (chiefly Jurassic). Ex. A. omata, 

 Inferior Oolite. 



Cirrus. Shell sinistral, conical or turbinate, or sometimes 

 nearly discoidal, with a very large umbilicus. Spire acute. Whorls 

 irregular, ornamented with strong transverse nodular ribs and finer 

 spiral ribs ; last whorl large. Aperture rounded, entire. Trias to 

 Inferior Oolite. Ex. C. ?wdosus, Inferior Oolite. 



Trochus. Shell conical, whorls numerous and flat or slightly 

 convex, spire sharp, interior nacreous ; base flat or nearly so, angular 

 at the periphery. Aperture entire, rhornboidal ; outer lip sharp, 

 oblique. Columella twisted, with a prominent anterior tooth-like 



