262 DEVELOPMENT OF CYPRAEA, ANOMIA, ETC. chap. 



the surface of the whorls, which are nearly enveloped by two 

 large lobes of the mantle, becomes overlaid with new layers of 

 shelly matter, until eventually the spire becomes embedded, and 

 ultimately disappears from view (Fig. 172). 



Patella^ when young, has a nautiloid shell (see Fig. 45, p. 

 134), but it is a remarkable fact that we are entirely ignorant, 

 in this commonest of molluscs, of the transition stages which 

 convert the nautiloid into the familiar conical shell. The young 

 shell of Pterocera% is deceptively unlike the adult, and is entirely 

 devoid of the finger-like processes which are so characteristic of 

 the genus (chap. xiv.). 



Among the bivalve Mollusca, Anomia in a young stage is 

 not to be distinguished from Ostrea. Soon a small sinus appears 



on the ventral margin, which 

 gradually deepens and, as the 

 shell grows round it, forms a 

 hole for the byssus, eventually 

 becoming fixed beneath the um- 

 bones (see Fig. 173). In Teredo 

 the two valves of the shell proper, 

 which is very small, become 

 ^ , . r ^. , lodged in a long calcareous tube 



Fig. 1^3. — Development of the byssus- t i i • i • 



or the plug-hole iu Anomia. (After or Cylinder, which IS generally 



open at both ends (see chap. xvi.). 

 In Aspergillum a somewhat similar cylinder is developed, but 

 the valves are soldered to the tube, and form a part of it, the 

 tube itself being furnished, at the anterior end, with a disc, 

 which is perforated with holes like the rose of a watering-pot. 

 In Clavagella the left valve alone becomes soldered to the tube, 

 while the right valve is free within it (see chap. xvi.). Fistulana 

 encloses the whole of its shell in a long tapering tube, which is 

 not at any point adherent to the shell. 



Terms employed to denote various Parts of the Univalve 

 Shell. — The apex is the extreme top of the spire, and generally 

 consists of the embryonic shell, which may often be recognised 

 by its entire want of sculpture. When the embryonic shell 

 happens to be large, the apex is often mammillated, e.g. in 

 Fusus^ Neptunea^ and some Turbinella ; in the Pyramidellidae 

 it is sinistral. 



The suture is the line of junction between any two successive 



