General Observations. 95 



But even with this disadvantage, I have so much confidence in 

 the permanency of those characters on which I propose to rest the 

 distinctions of the species, that I have ventured to believe some- 

 thing might be added to the present stock of conchological know- 

 ledge, by the publication of the few M'ell characterized species in 

 my possession. 



The idea of dividing the Genus into sections from the charac- 

 ters of the marginal ligament, has beei> borrowed from the article 

 on Conchology in the Supplement to the Encyclopaedia Britan- 

 nica. The obvious and convenient arrangement of the species it 

 affords, certainly entitles it to the highest consideration. 



The presence or absence of granulations, strias, and punctures ; 

 and the different modifications and arrangment of these on the 

 valves, are the most constant as well as the most obvious charac- 

 ters for specific distinctions. The next in importance to these I 

 consider to be the general formation and number of the marginal 

 teeth of the valves. On an exact coincidence in the number of 

 these in each valve through every individual of the same species, 

 I am not much inclined to rely ; nor should I feel at all disposed, 

 in the absence of other characters, to consider diiferences of this 

 nature sufficient for speci£c distinction. As far however as my 

 observations have gone, I have constantly found a peculiar forma- 

 tion of the marginal teeth accompanying other important and un- 

 questionable specific characters, and remaining constant in all ,the 

 individuals of each species in my possession. In the middle valves, 

 the number and peculiarity of formation of these teeth will pro- 

 bably be found more constant and regular than in the first and 

 last. 



The principal objection I am aware of to the employment of 

 such a character for specific distinction is this : — the marginal 

 teeth being entirely concealed by the ligament, it becomes neces- 

 sary to remove the latter before the species can be completely 

 ascertained.* In all cases it is desirable to simplify as much as 



* In dead specimens, a few minutes immersion in warm water will render 

 the marginal ligament sufficiently pliable to bear removal ? but care must be 

 taken not to injure the teeth, which are rather brittle. The best method is at 

 once to sacrifice a single specimen of cacli species; and, having completely 



