LAMELLIBRANCHIATA. 287 



and the external or fibrous layers, has long been recognised, and has 

 been forced, as it were, upon the notice of the palaeontologist by the 

 circumstance of the two being often separated from each other in 

 fossil shells, and sometimes from one having perished whilst the other 

 remained. As the nacreous layer alone forms the characteristic 

 hinge uniting the two valves of the shell, and alone receives the im- 

 pressions of the soft parts, the true characters of fossil shells, as those 

 of the genera Podopsis and Spherulites, which, in consequence of 

 their position in porous chalky beds, have lost all the nacreous layer, 

 cease to be determinable. When the inner layer is preserved, its 

 impressions reveal the organisation of the ancient fabricator of the 

 shell as clearly as do the forms and processes of fossil bones that of 

 the extinct vertebrate animal. 



The siphon in some of the elongated Inclusa cannot be retracted 

 into the shell ; they are consequently exposed, as in the Pholadomya 

 and Pholades : such species derive extrinsic shelter by burrowing in 

 sand or stone. The Pholades have supplemental calcareous pieces in 

 the hinge of the shell. The ClavagellcB and Aspergilla line their 

 burrows with a calcareous layer, which forms in the latter a dis- 

 tinct tube, closed at the larger extremity by a perforated calcareous 

 plate. One of the valves of the normal shell adheres to the tube 

 in the Clavagella^ and both are cemented to its inner surface in the 

 Aspergillum, 



In the Teredo, or Ship-borer, the most vermiform of molluscous 

 animals, the valves are reduced to mere appendages of the foot, at 

 one extremity of the animal, and are restricted in their function to 

 the operation of boring. As the ship-worm advances in the wood it 

 lines its burrow with a thin layer of calcareous matter. The length 

 of the body is chiefly due to the prolongation of the respiratory tubes, 

 each of which is provided with a small elongated calcareous triangular 

 plate. In the Teredo gigantea the tube, which sometimes surpasses 

 six feet in length, has parietes of from four to six lines in thickness, 

 the texture of which is crystalline or spathose. 



The valuable pearls of commerce are a more compact and finer 

 kind of nacre, often developed in the substance of the mantle, or 

 around a particle of sand or other foreign body which has gained 

 admission to the pallial cavity. The Meleagrina or Avicula mar- 

 garitifera of the Indian seas is most famous for these productions. 



The latest and best observations of naturalists and physiologists on 

 the sexual characters and generation of the Lamellibranchiata have 

 established the correctness of Leuwenhoek's conclusion that these 

 moUusca are of distinct sexes, some individuals being male and others 

 female. In the small species of Ariomia parasitic upon fuci on the 



