XVI PELECYPODA PROTOBRANCHIATA 447 



species, again, tend to burrow in such a way as to descend most 

 easily, and not be impeded by their own shells ; in other words, 

 they act as a wedge, and descend with their narrowest part fore- 

 most. But the burrowing organ, the foot, has to follow suit, and 

 gradually draws round to the narrowest part of the shell, so that 

 the habitual deep burrower, such as Lutraria, lies with its long 

 axis exactly at right angles to the surface, its siphons protruding 

 from, and keeping open, the uppermost or posterior margin of the 

 shell, and the foot producing the same effect upon the lower 

 or anterior margin. The deeper the burrower, the more elongated 

 does the shell become, imtil, through forms like Fholas and 

 Saxicava, we arrive at Solen, the most highly specialised burrower 

 of all, in which the breadth of the shell is equal throughout, 

 and no obstructive curve exists to impede its rapid ascent or 

 descent. 



The Pelecypoda have been classified in various ways ; by the 

 completeness or sinuation of the pallial line, depending on the 

 absence or presence of siphons, by the number of adductor 

 muscles, by the character of the hinge-teeth, and by the number of 

 the branchiae. For various reasons, none of these methods have 

 proved entirely satisfactory. That adopted here was suggested by 

 Pelseneer, and depends upon the character of the branchiae them- 

 selves, as suggesting successive stages of development (p. 166 f ). 



Order I. Protobranchiata 



Branchial filaments not reflected, the two rows inclined at a 

 right angle (more or less), ventral surface of foot more or less 

 flattened, byssogenous apparatus little developed, a single anterior 

 aorta, kidneys distinct, sexes separate, each genital gland open- 

 ing into the corresponding kidney. 



Faji. 1. Nuculidae.- — Labial palps very large, rows of branchial 

 filaments at right angles to one another, mantle edges open, 

 siphons contracted, foot disc-shaped, elongated ; shell equivalve, 

 oval, or produced, interior generally nacreous, hinge with numerous 



saw-like teeth. Silurian . Principal genera : Nucula (heart 



dorsal to the rectum) ; Palaeoncilo (Devonian), (?) Sarepta, Leda, 

 Yoldia, Mallctia; Tyndaria (Upper Tertiary), Zyroc^fsma (Silurian), 

 Actinodonta (Silurian), Bahinka (Silurian). 



Fam. 2. Solenomyidae. — Labial palps united, one row of 



