36 Dr. J. E. Gray on some Families of Bivalve Shells. 



one another, they are so exactly alike, that one is at a loss to 

 conceive what the difference between the two species can be. 



In making a long description of a species of a natural genus, 

 the characters which are common to the different allied s])ecies 

 must be repeated, and it is very diflBcult in reading the descrip- 

 tions, without a very accurate comparison, to seize on the essen- 

 tial character of the species under examination, and therefore it 

 is generally considered necessary to append to a long description 

 observations pointing out how the species described differs 

 from its allies, all of which trouble is avoided by a well-consi- 

 dered short character prepared after the examination and com- 

 parison of the allied species. 



On the other hand, a short analytic character, either of a ge- 

 nus or a species, is not so short and incomplete as it at first 

 appears ; for in examining and comparing a genus of shells with 

 the character, it should be compared first with the chai'acter of 

 the family, and then with all the sections and divisions until we 

 arrive at the generic character, and that character may be said to 

 contain the short essential character of the genus, combined 

 with the character of all the previous divisions and sections ; and 

 if these were written out together and repeated in each genus, 

 each of them would be found to be furnished with a character of 

 considerable length. It is exactly the same with the species. 

 This is the chief advantage of the analytic method of character- 

 izing the genera and species, that the characters common to two 

 or more genera or species need not be repeated for each. 



Fam. 1. Venerid^, Gray, Syn. B.M. 1842, 74. 



I propose to confine this family to the genera which have the 

 hinder lateral tooth compressed and forming a part of the mar- 

 gin of the shell, and the mantle lobes free. This will exclude 

 Cyprina, Petricola and Glauconome, which I believe form the 

 types of distinct families, and the genera Capsa and Diplodonta, 

 which I think ought to be removed to Tellinid^. The family so 

 restricted may be divided thus : — 



A. Foot lunate, inferior ; siphons united ; shell orbicular. 

 1. Dosinia. 2. Cyclina (Lucinopsis). 



B. Foot lanceolate anterior ; siphons partly united ; shell ovate, 



triangular or oblong. 



a. Anterior lateral tooth distinct ; cardinal teeth triangular ; shell 

 ovate. Meretricina. 



* Hinder cardinal tooth ci'oss- grooved or torn. 

 3. Meretrix. 4. Cuneus. 5, Grateloupia. 6. Trigona. 



fli 



