CONCHACEA. 119 



■ round an imaginary line, drawn througli the centre of the spiral 

 cone, down from the nucleus. When the edges of the successive 

 layers are marked by any external characters, the shell is said 

 to be concentrically striated, banded, grooved, costated, &e. A 

 fine illustration of the latter character is to be seen in the 

 Scalaria or "Wentletrap, fig. 351. Lines, bands, ribs, &c. in the 

 opposite direction, {transverse of some authors,) are " radiating " 

 in bivalves, as the ribs of Cardium, fig. 123, and "spiral" in 

 univalves, that is, following the direction of the whorls, as the 

 bands of colour in. Pyramidella, fig, 342. 



CONCHACEA. Bl. The eighth family of the order Lamelli- 

 branchiata, Bl. The shells are described as follows : nearly 

 always regular, valves closed all round ; apices curved towards 



. the anterior : dorsal hinge complete, with teeth and ligament ; 

 the latter external or internal, short and thick ; two distinct 

 muscular impressions, united at the lower part by a parallel 

 impression, which is frequently sinuated at the posterior. The 

 genera described in this family are divided into three sections. 

 First, those which are regular, and have distant lateral teeth, 

 Cardium, Donax, Tellina, Lucina, Cyclas, Cypriiia, Mactra, and 

 Erycina. Second, those which are regular, and have no distant 

 lateral teeth, Crassatella and Yenus. Third, those which are 

 irregular, Yenerupis, CoraUiophaga, Clotlio, Corbula, Sphsenia, 

 and Ungulina. 



CONCHACEA. Lam. A family of Lamarck's order Conchifera 

 Dimyaria. B-egular, unattached in general, closed at the sides. 

 They are always more or less inequilateral. The Marine Con- 

 chacea are those which inhabit the sea. The fluviatile Conchacea 

 are those which are found in rivers, ponds, &c. Each of these 

 contain various genera, which may be arranged as follows : — 



Fluviatile. 



1. CteenelI/A. Three cardinal teeth ; ligament long ; shell 

 thin. Eig. 114. 



2. Ctclas. Thin, oval ; cardinal and lateral teeth ; anterior 

 side shortest, including Pera. 



