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THE GENUS DOSINIA AND ITS SUBDIVISIONS. 

 Ey A. J. Jttees-Bkowne, F.R.S., F.G.S. 



Bead 12th April, 1912. 



The geuus Dosinia includes a large number of species, aud tliese 

 vary considerably both in external and internal characters. It is by 

 no means the compact geniis that the definitions given by Woodward. 

 Adams, and Fischer would lead one to suppose, for these definitions 

 do not apply to all the species. 



In his Catalogue of the Conchifera or Bivalve Shells in the British 

 Museum, Part I, Veneridoe, etc. (1853), Deshayes enumerated 

 85 species; Adams described several new species in 1855; liomer 

 again recorded others in 1860 and 1862, so that his monograph on 

 Dosinia (published in 1862) contains the names and descriptions of 

 105 species, notwithstanding the fact that he united some of the forms 

 which had been described under different names. 



In such an assemblage of species it is only likely that differentiation 

 should have produced several natural groups, and it is not surprising 

 to find that several authors have arranged the species in a number of 

 sections. Sowerby and Deshayes grouped them solely by the dift'erent 

 characters of the dorsal border, but though the importance of these 

 may be admitted, reliance on any one such set of characters does not 

 lead to a very natural arrangement. Sowerby made seven such 

 groups or sections, while Deshayes was content with five, which he 

 defined in Latin as follows : — 



1. Margine dorsali integro. [Xo escutcheon.] 



( 1 ) Strife simplices. 



(2) Strife ad latera scabrge vel lamellosae. 



2. Margine dorsali circumscripto. [A defined escutcheon.] 



(3) Area dorsali in medio prominente. 



(4) Area dorsali depressa, plana. 



(5) Area dorsali excavata. 



Homer in his monograph objects to Sowerby's divisions as being 

 unnatural, and himself proposes a series of eleven sections, but these 

 are no more natural or satisfactory than those made by Sowerby aud 

 Deshayes. Moreover, he gave no definitions of his sections, merely 

 indicating them by the name of a typical species, his groups being as 

 follows : — 



1. Sectio D. concentricae. 7. Sectio D. juvenis. 



The first of these sections is practically the same as those of Sowerby 

 and Deshayes, and is undoubtedly a natural group. The second is 

 also a natural assemblage of peculiar species which I have classed as 



VOL. X. — JUNE, 1912. 



