NAPLES FAUNA IN WESTERN NEW YORK, PART 2 223 



existence of an extensive byssus which would draw the shell close to the 

 base of fixation. With a shorter opening, the byssus, from analogy with 

 existing species, would be longer, and hence the flanges bounding the 

 opening, i. e. the walls of the lunule, may find opportunity to become more 

 horizontal in their direction. Such flattening of the valves on the byssal 

 side is frequently indicated even where there may be no specialized opening 

 for the extrusion of the byssus itself, e. g. the common Mytilus edulis 

 and probably its ancient allies passing under the name of Mytilus and such 

 other names as Mytilarca, Byssonychia, etc., where the abrupt antero- 

 ventral slope is that which is opposed to the surface of attachment. The 

 fact of this attachment is again indicated by the virtual absence of 

 hinge structure in all these shells and indeed throughout the lamelli- 

 branchs of this fauna. Granting that hinge structures are provided for 

 protection against movements of the water, dependence by attachment is 

 thus suggested by the absence of such structures. To the prevalence of 

 the structureless hinge in these mollusks we have already referred. It is to 

 be noted that when, in our observation, the two valves of these shells are 

 found spread wide open without attachment, coherence is maintained from 

 the beak along the edge of the lunule, while in Pterochaenia the reverse is 

 the case ; in both the opening taking place along the back of the umbo, 

 whether the direction of the beak be normal as in the latter, or reversed as 

 in the former. 1 



As to the structure of the hinge in this species, exact barite replace- 

 ments show that there is a total absence of inosculating denticles ; beneath 

 the beaks of both valves alike is a short, regular, triangular surface for the 

 ligament ; in front the hinge line runs directly into the lunular opening ; 

 behind, the valves interlock at the edges by the stronger development of 

 the first two or three radial plications. 



The larval shell or prodissoconch, which is well displayed in some of 

 our most delicate replacements, casts very suggestive if not important light 



1 See- Lunulicardium encrinitum, pi. 2, fig. 20, and Pterochaenia frag- 

 ilis, pi. 5, fig. 1, 2. 



