76 



Genus 2. HIPPOPUS, Meuschen. 



Animal; similar to that of Tridacna, but wanting a byssus. 

 Shell ; trapeziform, many-ribbed, fmbriately imbricated through- 

 out, and richly coloured ; lunule almost obsolete. 



Of the Hippoptis, or Horse-foot Clam, only a single species is known. 

 The animal is like that of Tridacna, except that it has no byssus, or only 

 the rudiment of a byssus, whilst the lunule of the shell is closed, or nearly 

 so. The shell, which is a common ornament in every house where shells 

 are collected or admired, is alike conspicuous for the beautiful symmetry 

 of its growth, the elegance of its sharply imbricated frilled sculpture, and 

 the richness of its colouring of crimson spots upon a yellow ground. 



Figure. 



Hippopus MACULATUS. PI. 30. Fig. 176. Shell, with one valve dropped 

 to show on one side the marginal ligament, on the other the hollow 

 serrated luuule. 



Order 2. BIMUSCULOSA. 



Shell ; attached to the animal by two distant lateral muscles. 



The remainder of the headless mollusks are for the most part of a more 

 oblong and even elongated growth, and the shells are attached to the ani- 

 mal by two adductor muscles, one at each end. The hinge is of a more 

 complicated structure, composed of teeth more strongly interlocking with 

 each other, and they have little of the fibrous laminar texture of so many 

 of the unimuscular order. 



Chamacea. 

 Naiades. 

 Trigonacea. 

 Arcacea. 



Cardiacea. 



CoNCHACEA. 



Nymphacea. 



LlTHOPHAGA. 



Mactracea. 



Myaria. 



Solenacea. 



Pholadaria. 



tubicola. 



Family 1. CHAMACEA. 



Shell ; irregular, inequivalve, affixed, with a single thick oblique 

 tooth in each valve, which is sometimes obsolete. 



