i68 Records of the Indian Museum. [Vol. XIX, 



Measurements of Shells {in millimetres). 



A. B. C. (Single left valve). 



Length 17-1 25-2 43-4 



Breadth io-2 I3"8 23'2 



Height 7 8-5 7-1 



Shells. No. M ^V-, M ^iiv-'S Zoological Survey of India {Ind. 

 Mus.). 



Relationships. Simpson's remark quoted already- regarding 

 the mollusc being doubtfully a Unionid is not justified in view ol 

 the specimens now discovered. The genus Arcidopsis has a super- 

 ficial resemblance to another Indian genus, Trapezoideus ,^\m^son, 

 but there does not seem to be any true relationship. It is, however, 

 impossible to discuss its true position until the anatomy has been 

 investigated. I do not agree with Haas {loc. cit.) in considering 

 A . footei as probably being congeneric with species like Trapezoideus 

 misellus (Morelet), for the specimens before me more distinctly show 

 that they do not belong to the genus Trapezoideus^ the resemblance 

 with this genus being purely superficial. 



2. LAMELLIDENS JENKINSIANUS (BENSON) AND ITS SUBSPECIES. 



In his catalogue of the Asiatic Naiades in the Indian Museum ^ 

 Preston described a new species of the genus Parreyssia, Conrad, 

 from a single dead shell from Dacca, Eastern Bengal. This form 

 he named P. daccaensis. His description of the species is ver}^ 

 short, being only a comparison with P. feddeni (Theobald), to which 

 he considered it to be closely allied. In his later work* in the 

 " Fauna" series he did not add anything to his original description, 

 btit published figures of the type-shell. luvSimpson's " Catalogue," ^ 

 which was published before the '^ Fauna " volume, Preston's ori- 

 ginal description is included without comment. 



Whilst identifying a small collection of Unionids made b}- 

 rayself and Babu D. N. Sen, of the Bengal Fisheries Department, 

 at Dacca and other places in the vicinity, I found on examining the 

 type-specimen of P. daccaensis that the shell did not belong to the 

 genus Parreyssia and that Preston was certainly mistaken in des- 

 cribing it as a new species of that genus. With the above-men- 

 tioned collection from the type-locality and other places in the dis- 

 trict^of Dacca, as also the large collections in the Indian ^luseum, I 

 have been led to the following conclusions : — (i) Preston's P. dacca- 

 ensis is a young shell of a highly peculiar but hitherto unrecognized 

 form of Benson's Unio jenkinsianus* (2) Benson's Unio jenkinsianus 

 is not a distinct species of the genus Lamellidens, as Simpson 

 doubtfullv believed, nor is it an abnormal form of L. marsinalis 



' Rec. Ind. Mus. VII, p. 300 (1912). 



■2 Faun. Brit. Ind. Fresliiv.-Moll. pp. 65, 66, figs. 16, 1—3 (1915, 



S Descr. Cat. Naiades, p. 1 1 14 (Detroit, Michigan, 1914). 



4 Ann. Mag. Nat. Nist. X, p. 185 (1862). 



