222 NEW YORK STATE MUSEUM 



or fusion of the adductor scars. Other specimens indicating internal 

 characters show but the single adductor scar [sec L. hemicardioides, 

 pi. 2, fig. 1 1-16] ; but we seem to have in L. m u 1 1 e r i a case of approxi- 

 mation of the muscles which may be compared to that of the genus 

 Tridacna, in which the two adductors have approached each other and 

 joined in the center of the shell, the byssus protruding from the anterior 

 part of the dorsal line. 



The second supposed function is improbable, as giving an external 

 ligament of a size and power relatively immense and impracticable to the 

 proportions of the shell. 



The third suggestion is entitled to some consideration for the reason 

 that all these Lunulicardia are thin shelled mollusks and are associated 

 almost without exception with other equally thin shelled organisms indicat- 

 ing in some measure a free swimming habit. In such conditions the shell 

 might be compared with a swimming aviculoid like Lima h i a n s, which, 

 though a byssus-spinning mollusk, has the power of breaking away from 

 its byssal nest and propelling itself by the extrusion of water from 

 between the valves. It would not be necessary to this conception of the 

 function of the Lunulicardium hiatus that the latter should be situated on 

 the anterior side of the animal. 



Considering the probability of the flattening being anterior and hence 

 a lunule, and of the hiatus being an opening for the byssus, we have these 

 features of importance : When the hiatus attains considerable length, it is 

 notable that the walls which bound it are narrow, sickle-shaped areas stand- 

 ing vertically to the horizontal axis of the animal. These surfaces we are 

 proposing to term the suae or steal surfaces. Only when the hiatus is 

 extremely short, that is to say, less than one half the hight of the valve, do 

 the sicae display a tendency to horizontal expansion. With such a hiatus 

 bounded by such vertical surfaces, it seems to us a rational proposition that 

 this structure could be brought about only by close attachment of the shell 

 to some substantial opposing object preventing growth toward the surface 

 of attachment. To fill so large an opening seems to us to necessitate the 





