224 Natural- Historical Collections. 



nacreous. They correspond to the nacreous pieces of the chitons, the arthrome- 

 ral and arthroceral pieces of the articulate animals. 



In the pholades we have the great advantage of seeing the pieces separated 

 from each other. In the chitons and patellse we have seen a shell constructed 

 vertically with respect to the arrangement of these pieces. Among the latter 

 there are several species in which the basilar piece tends to leave the apicial and 

 median point of the summit of the shell, to place itself upon one of the edges, so 

 that the shell is no longer regular in its different regions. In these tribes the 

 molluscous animal is merely covered by its shell ; but in the bivalve it is contain- 

 ed within its shell, the valves of which are frequently of the same extent and form. 

 These valves have the faculty of opening, which is effected by the equal separa- 

 tion of their free edges, and the moving of the valves upon their hinge. In the 

 chitons we saw shells whose different pieces are free at their lateral edges, (the 

 anterior and the posterior ;) while in the patellse these pieces are united together 

 to form a cone, which they necessarily do from their triangular form, their base 

 being broader than their summit. From this difference in the uses of the shell, 

 there results a great difference in the appearance of the relative position of the 

 pieces of the shell in the different mollusca. Thus the first layer of the shell of 

 a patella or a chiton is always superior, because the shell is always superior to 

 the animal. Sut this first layer appears altogether inferior in the bivalve mol- 

 luscous animal, which, in opening the two valves of its shell, is obliged to place 

 its summit or first layer below. Excepting in this appearance of difference, which 

 arises from the plane of position of the shell, the siiell of a bivalve molluscous 

 animal is nothing but the conical shield of a patella, of which the pieces of the 

 second and third layer are not united by their lateral edges, and roll upon the 

 piece of their first layer, which constitutes the hinge, or a portion of the hinge. 

 If the hinge of a pholas be placed uppermost, we shall have the true segment of 

 a chiton. 



In the shell so placed we can easily distinguish the pieces pointed out in the 

 preceding genera. The basilar layer of the chiton here forms a piece which is 

 often remarkable for the diversity of its position and its modes of development. 

 It is very broad in a small species named Ph. terebra. In almost all the spe- 

 cies it is open and as if split into two behind ; while anteriorly it commonly ap- 

 pears formed of a single piece. In the Ph. clavata, however, it is formed of two 

 very distinct pieces, and these pieces are less external, and penetrate into the ar- 

 ticulation. In the Ph. dactylus, these basilar pieces are also placed in the arti- 

 culation, form two pieces similar to the internal valves, and are stiU distinct from 

 the neighbouring pieces ; but in the Ph. dactyloidea they are with great diflS- 

 culty distinguished, being united to the other pieces of the shell; and in Ph. costata 

 they are united in their whole extent. This genus, therefore, proves that the 

 basial is originally composed of two distinct pieces. 



The second layer of the shell of a chiton here forms the large or external valves. 

 The two pairs of pieces which compose it may be independent of each other, and 

 of the neighbouring pieces, as is evident in most of the pholades. The first pair 

 of pieces of this layer, the costal pieces, forms the median portion of the valves, 

 as may be seen in Ph. clavata and crispata. In the latter they form half of the 

 valve, and in the costata nearly the whole. The second pair of pieces, the poler' 

 gal, which, like the first, arise from the vicinity of the hinge, but more anterior- 

 ly, and from the anterior edge of the costal pieces, form the whole anterior and 

 prolonged part of the outer valve. ]\Iost commonly the striae or wrinkles of 

 these polergal pieces do not affect the same direction as that of the costal. 



Thus the large valves of a pholas are chiefly formed by the two pairs of this 

 layer. We have seen that the basial piece may be united to them at the hinge ; 

 and we shall presently see how far the arthromeral pieces may be annexed to them. 

 In the Ph. cristatu the costal pieces behind and the polergal pieces before seem 

 to form almost ths whole of the valves, as may be more easily seen from the cir- 

 cumstance, that their calcareous fibres descend in the same direction. 



The two pairs of pieces of the third layer of the chitons, which are internal in 



