466 LUCINACEA. 



Syn. Pelvis, Miihlf. Cerceis, Gist. 



Ex. H. equinus, Meuschen, pi. 113, fig. 3. Shell, H. 

 equinus, fig. 3, a, 3, b. 



This genus is readily distinguished from the precediug 

 by the absence of the byssal foramen, and by the double 

 primary tooth of the hinge. The only species known is 

 from the Indian Seas, where it inhabits the coral-reefs at 

 low-water mark. 



Order LUCINACEA. 



Mantle-lobes free beneath, united posteriorly, forming a 

 separate siphonal opening. Foot usually cylindrical, elon- 

 gate, inferior. Anterior adductor muscle usually elongate. 



Fam. LUCINFDtE. 



Labial palps small and rudimentary. Mantle-lobes free 

 beneath, furnished behind with one or two sessile siphonal 

 apertures ; gills one on each side, large, oval, thick. Foot 

 cylindrical, elongate, inferior, usually hollow throughout 

 its entire length, the tube opening into the spaces of the 

 visceral cavity. 



Shell equivalve, free, regular, closed, more or less orbi- 

 cular. Hinge and sculpture of surface variable; interior 

 dull, or obliquely furrowed; ligament external, or sub- 

 internal. Muscular impressions very large, rough, elon- 

 gated ; pallia! line simple. 



Genus LUCINA, Bruguiere. 



Siphonal orifices simple, without a prolonged anal tube. 

 Shell orbicular, compressed ; surface of valves variable ; 

 lunule distinct. Hinge usually composed of two diverging, 



