CONCHIFERA. 121 



Genus 4 Wi KhMk,-- Lamarck. 



Generic Character. — Body enclosed in a shell ; two op- 

 posite retractile, rather large fins, inserted at the sides of 

 the mouth ; head indistinct ; mouth terminal, placed at the 

 junction of the tins ; no eyes ; branchiae lateral. Shell 

 horny, transparent, oval-globular, with three teeth behind, 

 open at the summit and the two posterior sides. 



Hyalcea tridentata The Three-toothed Hyal^a. 



Plate XIV. fig. 12. Transparent, horn-coloured, globular; 

 posteriorly tridentate ; summit and two posterior sides open ; 

 very finely striated transversely. Three-fourths of an inch 

 long. Inhabits the Mediterranean, and the seas of warm 

 climates. 



CLASS II.-CONCHIFEEA. 



Animals soft, inarticulated, destitute of head or organs of 

 vision, and always fixed within a bivalve shell ; having ex- 

 ternal branchiae, situated between the body and the mantle, 

 which is very large, and forms two thin lobes, enveloping 

 the whole body ; a simple circulation, and a unilocular 

 heart. 



All the animals of this class live in the sea or in fresh 

 water. None of them have an internal shell. The body 

 is invariably soft, and the mouth is situated near the hinge 

 anteriorly. 



Lamarck divides this class into the two following orders : 

 — I. MoNOMYARiA, with One muscle of attachment; shell 

 exhibiting interiorly but one subcentral muscular impres- 

 sion — II. Dim Y ARIA, having at least two muscles of at- 

 tachment; the shell exhibiting interiorly, two separate, 

 lateral, muscular impressions. 



L 



