242 MOLLUSCA. GASTEROPODA 



Solarium. Shell conical, depressed, angular at the periphery. 

 Aperture entire, sub-quadrate ; lip sharp. Umbilicus wide and 

 deep, limited by a sharp edge which is generally crenulated. Oper- 

 culum horny, spiral. Jurassic to present day. Ex. S. perspectivum, 

 Eecent ; S. canaliculattim, Barton and Bracklesham Beds. 



Purpuroidea. Shell thick, oval, spire rather short, last whorl 

 inflated. Whorls step-like, flattened below the suture, with tubercles 

 or spines at the angles. Aperture with a small notch anteriorly ; 

 outer lip thin. Inferior Oolite to Upper Cretaceous. Ex. P. 

 nodulata, Great Oolite. 



Littorina. Shell thick, without a nacreous layer, turbinate, 

 with few whorls, without umbilicus. Aperture rounded, angular 

 behind, outer lip sharp. Columella flattened. Operculum horny, 

 paucispiral. Lias to present day. Ex. L. littorea, Bed Crag to 

 present day. 



Capulus. Shell conical, with apex bent considerably backward 

 and more or less spirally inrolled. Aperture rounded or irregular. 

 Muscular impression horse-shoe shaped. Lower Palaeozoic to present 

 day. Ex. C. hungaricus, Coralline Crag to present day. 



Platyceras. Allied to Capulus ; apical part usually more ex- 

 tensively coiled, dextral. Surface smooth, or with concentric striae, or 

 radial folds or spines. Cambrian to Trias. Ex. P. comutum, Silurian. 



Calyptraea. Shell thin, conical, trochiform, spiral, apex 

 central ; interior with a spiral plate under the apex and attached 

 at the periphery. Aperture nearly circular. Cretaceous to present 

 day. Ex. C. chinensis, Coralline Crag to present day. 



Natica. Shell oval, globular, generally smooth, spire short, 

 last whorl very large. Aperture semi-lunar or oval, entire ; outer lip 

 sharp, inner lip thickened with callus, not crenulate. Umbilicus 

 usually present, often filled with callus. Operculum of the same 

 size as aperture, horny or calcareous, paucispiral, nucleus excentric. 

 Trias to present day. Ex. N. canrena, Eecent ; N. millepunctata, 

 Coralline Crag to present day. There are numerous sub-genera. 



Xenophora (= Phorus). Shell conical, low, with flattened or 

 concave base ; periphery of last whorl sharp. Aperture large, oblique, 

 lower part concave, outer lip sharp and oblique. Umbilicus generally 

 small. Whorls flattened, covered with agglutinated foreign bodies. 

 Cretaceous to present day. Ex. X. agglutinans, Barton Beds. 



