34 



suited in greater knowledge of the internal arrangements 

 of the types of these groups, and greatly modified the for- 

 mer method of classification of these animals, which was 

 based chiefly on external characters. Had Mr. Meek not 

 been prevented from extending his investigations by the 

 melancholy and long-continued illness which terminated in 

 his death, it is very probable that he would have reconsid- 

 ered his earlier views. At all events, it is evident that cer- 

 tain differences exist between the above-mentioned groups, 

 ■which are apparently as important as many which have 

 formed the basis of accepted genera, although it cannot be 

 denied that a general and rather gradual transition may be 

 traced between the different sections of this family. These 

 it appears better, for the sake of discrimination, to accept 

 for the present, leaving for the future the task of thorough 

 correlation and subordination. 



The preceding is but the beginning of these complications. 



In 1828, Dalman had described the genus Leptaena {q. v.) 

 ■with four species (and references to others which he sup- 

 posed to belong to it), which formed a heterogeneous assem- 

 blage. Of the four species from which Dalman's type must 

 be selected, L. euglypha, the third, is a typical Strophomena 

 (Blainville), agreeing with 8. rngosa, Bl. ( = planumhona, 

 Hall), in having the dorsal (or haemal) valve convex, while 

 the ventral or socket valve is concave externally. The first 

 and second, L. ntgosa, Dalm. (= rhomhoidalis) , and depressa, 

 Dalm., are, according to Mr. Davidson, mere varieties of one 

 species. This he includes in Strophomena, not considering 

 the differences by -which it is distinguished from the type 

 of the genus Strophomena (Blainville) to be of generic or 

 subgeueric importance. These differences are chiefly that 

 the relative convexity and concavity of the valves are re- 

 versed, and that certain minute differences in the form of 

 the muscular and vascular impressions, in the grooving of 

 the tips of the cardinal process, or of the hinge-line, have 

 been noticed, and also some differences of form, apparently 

 owing to the general convexity, or to the sharp geniculation 

 of the valves in various species. The fourth species, L. 

 transversalis, Dalm , exhibits differences which seemed suf- 



