1920.] B. Prashad : Notes on Lamellibranchs. 169 



or L. corrianus as Hanley and Theobald' considered the unique type 

 before them to be. It is indeed a form closely allied to L. margin- 

 alis through Hanley and Theobald's var. obesa. I cannot agree 

 with Preston in regarding L. jenkinsianus as a subspecies of L. 

 viarginalis, and (3) the form Uiiio niarginalis var. obesa of Hanley 

 and Theobald must be considered as a species distinct from L. niar- 

 ginalis on both anatomical and malacological grounds. It is a 

 large, rather thick-shelled form which appears to be very like the 

 ancestral form, from which the thick-shelled forms obesa ^ jenkin- 

 sianus and daccaensis have been evolved. It shows onh* a slight 

 modification from the closely allied L. marginalis , with which it 

 has up to now been confused. 



The ancestral type of shell in this group of series is the form 

 to which Hanley and Theobald gave the name obesa in 1876. 

 They did not properly describe the form but published the follow- 

 ing note in the explanation of their plate; — "A giant form, 

 which does not exhibit the ochraceous band, and is peculiarly 

 swollen. It comes between the var. lata and the typical form. 

 The upper anterior tooth is almost linear ; the lateral are not bent 

 at the extremity and the upper one in the left valve is scarcely 

 developed." Their figure of the shell imperfectly shows the 

 hinge in the right valve only, but this and the outline and form 

 of the shell as shown in the full-size figure of the left valve are 

 quite enough for distinguishing the form. Neither Preston nor 

 Simpson added anything to the above meagre description. Ben- 

 son's name jenkinsianus was given fourteen 3'ears earlier, but the 

 form to which he assigned this name is onh^ subspecifically 

 distinct from Hanley and Theobald's obesa. Under the circum- 

 stances our only course is to describe obesa as a subspecies of jen- 

 kinsianus, which, owing to the priority of Benson's name, has to 

 be taken as the name of this group of forms. Preston's name 

 daccaensis is retained for the rather peculiar subspecies noted 

 already, as a ^-oung shell of it was given this name. The relation- 

 ships of these forms with one another and of the group as a whole 

 with L. marginalis may be expressed as follows: — 



L. jenkinsianus subsp. daccaensis. 



I 



L. jenkinsianus. 



I 



L. 'jenkinsianus subsp. obesa. 



L. marginalis. 



In a recent paper"^ Dr. Annandale and I described a Unionid 

 from Seistan under the name L. marginalis subsp. rhadmaeus. 



^ Conch. Ind. p. 19, pi. xli, fig. 4 (1876 ). 



2 Rec. Ind. Mas. XVIII, pp. 59—62, fig. 9 (A,B), pi. iii, figs. 9, 10; pi, 

 viii, figs, 7 — II (1919), 



