STROPHODONTA. ' 7# 



a process extending therefrom, which, in its indented front margin, 

 leaves a space for the protrusion of a minute pedicle. 



The question of the presence or absence of a foramen, or the smooth 

 or crenulated hinge-line, might not alone or together be of sufficient 

 importance to indicate a distinct genus ; but taken collectively with the 

 form of the muscular impressions and other points, we find so wide a 

 difference from the typical species of Strophomena, that it appears to me 

 the separation is required. Were we to unite typical Strophomena with 

 the forms I have designated Strophodonta, we could, with equal pro- 

 priety, unite many of the Streptorhynchus in the same group.* . 



The distinction among the genera or subgenera of the strophomenoid 

 shells does not, in all cases, appear to be satisfactorily determined; and •- 

 we may be often disposed to regard a newly-observed feature as characte- 

 ristic and reliable for generic determination, when the same may be 

 found in species where the associated characters are incompatible. 

 Among other examples we find one or more forms with crenulated hinge- 

 line, where the area is well defined on both valves, with the triangular 

 foramen closed by a convex deltidium, but otherwise resembling Strep- 

 torhynchus. 



It may be doubtful how far the presence or absence of dental lamellae 

 should influence our considerations in determining the generic relations ; 

 and whether the form of the cardinal process, its extension backwards 

 or inclination forwards, should influence the decision. 



Among the forms which I have designated Strophodonta, we have a 

 group of species, following the typical form in their internal characters, 

 in which we may enumerate S. demissa, S. inequiradiata, S. cmcava, S. 

 hemispherica, S. nacrea, and some others. 



. In another group of nearly flat species, we find the partial or entire 

 absence of foramen" and dental lamellaa, with broad spreading muscular 



•In the Canadian Journal of July 180], Mr. BiLiiitas. in endeavoring to prove the identity of Stro- 

 PBODOHTA and Stroi'DOhkna, has given as an illustration of the latter genus the S. filitexta. which is a 

 Stbeptorhynchi-s. If the cJiaractcrs of the recognized Genera Steophomena and Streitobuynchus are 

 go similar as to deceive this acute observer, it may be supposed that differences of equal value between 

 Stbophodosta and STRonioMENA may have escaped notice. 



