LAMELLIBRAKCHIATA. 1 7 



The distinction between the internal or nacreous layers, and tlie 

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

 forced, as it were, upon the notice of the palaeontologist by the cir- 

 cumstance 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 AviculidtB and RadioUtes, which, in consequence of their 

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

 to be determinable, save when a natural mould of the interior has 

 been formed before the pearly lining of the shell was dissolved. 

 When the inner layer is preserved, its impressions reveal the organ- 

 isation of the ancient fabricator of the shell as clearly as do the forms 

 and processes of fossil bones that of the extinct vetebrate animal. 

 The layers of the thick subnacreous inner substance of the shell of 

 the Spondylus have frequently wide interspaces, called from their 

 contents " water chambers : " this " camerated " structure is well 

 shown in the right or lower valve of Sp. varius.* 



The siphon in some of the elongated Inclusa cannot be retracted 

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

 and Pholas : such species derive extrinsic shelter by burrowing in 

 sand or stone. The Pholades have supplemental calcareous pieces in 

 the hinge of the shell. Two small plates protect the umbonal 

 muscles, and a long narrow plate fills up the dorsal interspace of the 

 valves. The Clavagell^ and Aspergilla line their burrows with a 

 calcareous layer, which forms in the latter a distinct 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 navalis the valves are reduced to mere appendages of the 

 foot, at one extremity of the animal, and are almost restricted in 

 their function to the operation of boring. As the ship-worm ad- 

 vances 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 paddle-shaped 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. Two tubes are developed within its 

 siphonal end. In Teredo norvegica the tube is divided longitu- 



* CCCXVIIL p. 407. 



I. L 3 



