CONCHIFERA. 455 



Type, C. Hibernicum, PI. XIX., Fig. 5. C. aliforme, 

 Fig. 254. 



Shell, equivalve trigonal, conical and gaping infront, truncated 

 behind, with a long siphonal tube near the umbones ; anterior 



Fig. 251. Conocardiuni aliforme, Sby. Carb., Ireland. (Mus. Tennant.) 



slope radiately, posterior obliquely striated ; margins strongly 

 crenulated within ; hinge with anterior and posterior laminar 

 teeth ; ligament external. 



The truncated end has usually been considered anterior, a con^ 

 elusion which seems incompatible with the vertical position and 

 burrowing habits of most free and equivalve shells ; if compared 

 with Adacna (Fig. 253) the large gape (a) will be for the foot, 

 and the long tube (s) siphonal. C. Hihernicum has an expanded 

 keel, like Hemicardium inversum. The shell-structure is pris- 

 matic-cellular, as first pointed out by Sowerby ; but the cells 

 are cubical, and much larger than in any of the Aviculadce. In 

 Cardium the outer layer is only corrugated or obscurely pris- 

 matic-cellular. 



Fossil, 30 species. TJ. Silurian — Carb. North America, 

 Europe. 



Family XI. — Lucinid.e. 



Shell orbicular, free, closed ; hinge-teeth 1 or 2, laterals 1 — 1 

 or obsolete ; interior dull, obliquely fiuTowed ; pallial line 

 simple ; muscular impressions 2, elongated, rugose ; ligament 

 inconspicuous or sub-internal. 



Animal with mantle-lobes open below, and having one or 

 two siphonal orifices behind; foot elongated, cylindrical, or 

 strap-shaped [ligulate), protruded at the base of the shell ; gills 

 one (or two) on each side, large and thick, oval ; mouth and 

 palpi usually minute. 



The LucinidcB are distributed chiefly in the tropical and 

 temperate seas, upon sandy and muddy bottoms, from the sea- 

 shore to the greatest habitable depths. The sheU consists of two 

 distinct layers. 



Fig. 255 represents the animal of a species of Diplodonta, 



