6 CRETACEOUS PELECYPODA 



The principal variations in the form of tlie shells have been already alluded 

 to in the account relating to the general form of the body. In the typical forms 

 there are always two valves present whicli correspond to the sides of the body, and 

 into which all the soft organs of the animal are retractible. Tlie PROLADA CEA 

 form in part an exception of this, the true valves being sometimes reduced to a very 

 small size, while the posterior part of the body with the siplions is greatly pro- 

 longed and secretes a special calcareous tube. In all the other orders of the Pele- 

 cypoda the valves are, when free, either perfectly equal or sub-equal ; when one of 

 the valves is permanently attached, the free valve is mostly somewliat smaller, 

 while the other grows somewhat irregularly, adapting itself to the object upon 

 Avhich it is sessile. The external ornamentation of these foreign objects is, however, 

 generally transferred to one or both the valves of the sessile shell. 



The principal increase of the valves takes place at their periphery, the shell 

 mass being secreted by the edges of the mantle, which when entire produce a simple 

 striation, more or less regularly concentric with reference to the position of the 

 umboues ; but when they are undulating or dentate, the formation of various 

 radiating striae and ribs on the shell surface is the consequence. Thus, consider- 

 ino- these two kinds of secretion, we shall have naturally to divide the ornamenta- 

 i ion of the shell-surface of Pelecypoda into a concentric and a radiating one ; the 

 other terms relating to stria3, ribs, spines, tubercles, &c., are only relative and 

 easily intelligible ; they refer more to the character of individual specimens than 

 to that of the larger and more general divisions. 



The terminology of the parts depends on the position in which the shell is 

 placed ; and in regard to this important point two views are upheld by naturalists. 

 The one adopts the position, with the beaks laterally, the anterior end turned 

 below, and the posterior, or siphonal, above,- the other, the position with the beaks 

 uppermost, and the two ends anterior and posterior, as the more natural one. I 

 shall conform to the latter view which, though the older one, certainly appears 

 to me the more correct and more natural one, at least as far as the greater 

 number of the typical groups of Pelecypoda is concerned. 



The shell being placed in such a position that the umbones are situated above 



the oral end anteriorly and the siphonal end posteriorly will define our terminology. 



The shells will, therefore, be classed first as eqvA- or incqui-lateral. It is usual that 



the umbones are incurved towards the anterior end, but opposite cases are not 



uncommon. The margin of the shell just below and in front of the umbones is called 



the lunidar, that behind the beaks, Avhere the ligament is attached, if external, 



the areal. If the regions of these two margins are particularly circumscribed by 



lines or ribs, being cither peculiarly excavated or elevated, the names lunula and 



area are commonly applied to them. The ventral margin becomes in the above noted 



position of the shell the lower, and is naturally so ; at the anterior part of it the 



" foot protrudes, and this is, as a rule, much more often the natural position of the 



Pelecypoda, when moving in the mud or sand, than that the anterior part should 



