belonging to the Class Palliobranchiata. 37 



American writers prefer Rafinesque' s to Dalman's. Be this as it 

 may, it is not my intention to adopt the one in preference to the 

 other, as it is my opinion that both names can be advantageously 

 retained. 



The genus Leptcena, as constructed by Dalman, evidently in- 

 cludes two different groups of species, Leptcena depressa and L. 

 rugosa constituting the one, L. transversalis and L. euglypha the 

 other. These were the only species known to Dalman ; since his 

 time several others of both divisions have been discovered. 



It is now difficult to say which species Rafinesque considered 

 as typical of his genus ; our only alternative is then to ascertain 

 the type of Dalman's. The committee to whose labours I have 

 already been indebted, state, that " when authors omit pointing 

 out the type of the genus, it may still in many cases be correctly 

 inferred that the first species mentioned on their list, if found 

 accurately to agree with their definition, was regarded by them 

 as the type*." As Leptcena rugosa answers in every respect to 

 these terms, it follows that this species ought to be regarded as 

 the type of the genus ; and considering the claims which Rafi- 

 nesque' s name has to priority, we are to a certain extent war- 

 ranted in applying the name Strophomena to the group repre- 

 sented by Leptcena transversalis and L. euglypha f. 



It will now be necessary to point out the differences between 

 Leptcena and Strophomena. Both valves of Leptcena are more or 

 less wrinkled transversely : when the shell is young they are flat ; 

 afterwards their frontal margin becomes inflected, which is per- 

 manent in the dorsal or deltidial valve, but evanescent in the 

 opposite one, as its front soon becomes acutely deflected or folded 

 upon itself outwardly : by this means the frontal margins do not 

 meet each other, as in the Terebratulas, and as they at first 

 affected, but the anterior part of the upper valve overlaps that of 

 the under one, the inner surface of the one facing that of the 

 other at the same time. On the other hand, Strophomena has 

 plain valves, that is with reference to the wrinkles, and it is 

 in general regularly concavo-convex J, the convexity usually 



* Report of the British Association for 1842, p. 111. 



t If it cannot be ascertained which species Rafinesque considered as the 

 type of Strophomena, the S. rugosa figured in Blainville's ' Malacologie ' 

 ought to be looked upon as the typical one ; and in this case we are bound 

 to adopt Rafinesque's name, inasmuch as this species belongs to the group 

 represented by Leptcena transversalis and L. euglypha. 



% Strophomena and Orthis merge into each other by means of their flat 

 species. Fischer de Waldheim, in proposing the genus Orthotetes, has evi- 

 dently had in view some of these merging forms ; but the fact of Dal man's 

 typical species of Orthis (0. pecten) being also apparently the type of 

 Fischer's, the former genus necessarily falls to the ground. What is the 

 genus Hipparionyx of Vanuxem ? 



