CVTHEREA. 



into groups according to their peculiarities, if we would 

 arrive at any thing like precision in describing the charac- 

 ters by which each Genus is to be distinguished ; it would 

 perhaps be found necessary to constitute and characterize 

 several new genera; but this, we think, would be a decided 

 advantage to the science, particularly when the great 

 number of species, and the great diversity of character 

 among the species that are now arranged together under 

 those two generic appellations are taken into the account. 

 In order to convey a tolerably accurate notion of Cytherea 

 as it now stands, we shall endeavour to point out the va- 

 rious types of form of which it is composed. 



First, The typical species, C. lusoria, petechialis, and 

 others, are smooth on the outside, and covered with a thin 

 horny epidermis; there are three diverging hinge teeth in 

 each valve, and a lateral anterior tooth under the lunule, 

 which is elongated, and but indistinctly marked : there is 

 also a small rounded sinus in the muscular impression of 

 the mantle. Their general form is obtusely triangular, the 

 anterior side being rather shorter, and the beak distinctly 

 inclining forvt^ards. 



The second group of which Cytherea consists, and of 

 which we believe C. Corbis, Lam., is one, has four or five 

 diverging hinge teeth ; a more lengthened, almost lamellar 

 anterior lateral tooth ; a larger rounded sinus in the mus- 

 cular impression of the mantle ; a lengthened and still in- 

 distinctly marked lunule , a smooth outside, and, in such 

 species as we have been able to examine, a thin velvety 

 epidermis. These are more distinctly triangular, and the 

 beak less inclined forwards. 



A third group, consisting of C. Chione, Erycina, and 

 others, which are of a nearly regular oval form, having 

 the anterior side much shorter than the posterior : they 

 are smooth on the outside, have a thin horny epidermis, 

 and some of them are longitudinally grooved : they have 

 three diverging cardinal teeth in each valve, and a closely 

 approximated, short, blunt lateral tooth. The sinus in the 

 muscular impression of the mantle is very large, and ge- 

 nerally pointed at its anterior end. 



The fourth group are nearly lenticular in general form 

 and concentrically grooved on the outside ; the sinus in 

 the muscular impression of the mantle is large, oblique, 

 and straight sided j there are three hinge teeth in each 



