6 CEETACEOUS PELECYPODA 



The principal variations in the form of the shells have heen already alluded 

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

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

 into which all the soft organs of the animal are retractible. The FROLABA 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 siphons 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 perfe!?tly equal or sub-equal ; when one of 

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

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

 which 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 

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

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

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

 tion of the shell-surface of Pelecypoda into a coficentric and a radiating one ; the 

 other terms relating to striae, 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 Ihells will, therefore, be classed first as equi- or inequi-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 lunular, that behind the beaks, where the ligament is attached, if external, 

 the areaL If the regions of these two margins are particularly circumscribed by 

 lines or ribs, being either 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 loioer, 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 



