242 PEOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



be separated from lenticular is and quadrangular is merely because tbere 

 is sometimes a stronger ridge in this place than the other species 

 exhibit ? 



In all the essential characters of sculpture, strength of hinge- 

 plate, shape of teeth, and pallial sinus V. exalbida differs from 

 Katelysia proper, and agrees with V. lenticularis . More particularly 

 they agree in having only one grooved tooth in each valve, the posterior 

 right and the median left, both the median right and the anterior left 

 being narrow, entire, tall, and sharp, though at the same time slightly 

 rugose. 



There is nothing whatever in V, exalbida to afford any ground for 

 separating it from V. lenticularis ; consequently I place it in Dr. Ball's 

 Samarangia, and adopt that name for a group which is practically the 

 second section of Eomer's Katelysia. The hinge of these shells is not 

 that of Marcia, and I think they must be regarded as forming a distinct 

 genus. "With them, however, I am inclined to place the Eocene 

 fossils which were first separated by M. Cossraann in 1886,^ and now 

 bear the name of Mercimonia (Dall, 1902). These shells also have two 

 entire teeth in each valve, and the left median has a ledge on the 

 anterior side which seems to correspond with the laterally grooved 

 tooth in the left valve of Samarangia. 



In his description of the American Veneridaj Dr. Dall places two 

 other recent species in the restricted section of his genus ' Marcia ' 

 along with V. exalbida? These are V. Kennerleyi, Carpenter in Reeve, 

 and V. rufa, Lam., better known as V. opaca, Sow. I have not been 

 able to obtain or see a specimen of the former, so cannot discuss it, but 

 I have examined V. rufa, which is a thick oval shell having many of 

 the characteristics of Samarangia, but a somewhat different hinge. 

 The posterior part of the hinge-plate is deeply excavated, so that in 

 the right valve the posterior cardinal is very short, being abruptly 

 truncated by the border of the plate ; in the left valve also there is 

 a similar abbreviation of the plate, so that both the posterior and median 

 teeth are short, the latter being grooved in the middle, not at the side. 

 These differences seem to be at least of sectional importance, but at 

 present I refrain from proposing a new name for this single species. 



Another shell which looks from Dr. Dall's figure as if it might belong 

 to Samarangia is that described by him as dementia solida (op. cit., 

 p. 401). Its hinge differs considerably from that of dementia, and 

 seems to resemble that of V. exalbida. 



II. Systematic Definition of the Groups. 



Having completed the analytical part of my enquiry into the history 

 and characters of these shell- groups, I come now to the synthetical part, 

 i.e., that of estimating their relative taxonomic value, and of compiling 

 descriptive definitions of the genera and their subdivisions. 



In the first place, however, something should be said about the 

 moUusca to which the shells serve as coverings, and it is to be regretted 



1 Ann. Soc. Roy. Malac. Beige, vol. xxi, p. 106. 



2 Proc. U.S. Nat. Mus., 1902, vol. xxvi, pp. 396, 397. 



