262 DEVELOPMENT OF CVrRAEA, ANOMIA, ETC. chap. 



surface of the whorls, which are nearly enveloped hy two large 

 loljes of the mantle, becomes overlaid with new layers of slielly 

 matter, until eventually the spire becomes embedded, and ulti- 

 mately 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 sliell. The young shell of 

 Pteroceras is deceptively unlike the adult, and is entirely devoid 

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

 genus (chap. xiv.). 



Among the l)ivalve 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 

 l)ec()niing fixed beneath the um- 

 bones (see Fig. 173). In Teredo 

 the two valves of the shell proper, 

 whicli is very small, become 

 lodged in a long calcareous tube 

 Fig. 173.— Development of the byssus-or or Cylinder, whicli is generally 



plug-hole ill ^4«y//t/ff. (After Morse.) , i ,i ^ / m • \ 



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

 In AsjJergilhivi 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 disk, which 

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

 ClavajeUa the left valve alone becomes soldered to the tulje, 

 while the right valve is free within it (see Chap. xvi.). Fistula na 

 encloses the whole of its shell in a long tapering tulte, 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 mannnillated, e.(j. in Fusus, 

 Neptunea, and some TurMnella ; in the Pyramidellidae it is 

 sinistral. 



The suture is the line of junction lietween any two successive 



