Anatomy of the Lamellibranchiata. 581 



other ; the foot is developed into an organ of different forms 

 and functions ; the mantle commonly has orifices or tubes 

 for the inlet and exit of the water. 



Some Lamellibranchiata, from their elongated form, may 

 be said to be allied to the Annelida. From the description 

 of M. Deshayes,* the animal of the Dentalium is intermedi- 

 ate in its position between the Teredo and the Serpulacece. 

 In some Pholades we see a tendency of the valves to separate 

 at the insertion of the siphons, and in the Teredo, &c, we 

 find two small anterior valves, encased in a long calcareous 

 tube. Of the Gastropoda, the Chiton and Patella come near- 

 est to the Lamellibranchiata in structure; the valves of the 

 former seem analogous to the dorsal lamina of the Pho- 

 lades, whilst the shell of the Patella, Emarginula, &c, ap- 

 pears to be formed of the two united valves of the ordinary 

 Lamellibranchiata. 



The Brachiopoda seem to be most naturally placed between 

 the Lamellibranchiata and the Cirrhopoda ; and considera- 

 ble analogy seems to exist between the Rudista of Lamarck, 

 and some of the sessile cirrhopodous animals. t 



The valves increase in size by the addition of new layers, 

 each secreted within the preceding one, and projecting be- 

 yond it. Their shape is determined solely by that of the man- 

 tle, which secretes them. Their degree of vertical convexity 

 commonly diminishes at the lower edge, or last-formed part 

 of a valve; however occasionally, as in some varieties of My- 

 tilus, the reverse takes place. 



In the opposite direction, likewise, the valves present dif- 

 ferences in the degree of convexity. Thus an impression (lu- 

 nule) often exists anteriorly before the beaks ; and a similar 

 one is sometimes present behind them, {Jissure), supporting 

 two prominences, (nympha), to which the cartilage is com- 

 monly attached. The calcareous layers of the valves are 

 sometimes at a distance from each other ; in this case we oc- 

 casionally see a fluid interposed between the last of the lay- 

 ers ; and between the preceding ones masses of carbonate of 

 lime of an uncrystallized appearance are seen, which appear 

 to enter the cells by a transudation.! These internal layers 

 appear to be secreted by the corresponding central portion 

 of the lobes of the mantle. In some of the fossil genera, one 

 of the valves takes an elongated and multilocular form. The 



* Blainville's Malacologie. 

 f Desmoulins, sur les Sphcerulites, Isis, 1829. 

 I In the Cardita concamerata, Sowerby describes a sort of cup in the in- 

 terior of each valve, which he supposes contains the ovaries. Do the inter- 

 nal processes of the Photos correspond to these ? ' Zool. Journ vol. ii. 



