SPECIES OF PISIDIUM. 19 



shells, give place to an elongate oval with comparatively central 

 urabo (PL V, f. 126 & d) or to an elongate form with posteriorly 

 placed umbo as in var. elongata (PI. V, f. 16), or assume a trigonal 

 outline as in var. nova (PL V, f. 17 d) and certain examples from 

 the Pleistocene of Grays (PL VI, f. 4 Tc, t, u ; IX, f. 9). Normally 

 it is a fairly tumid shell (PL V, f. 1 & 2), but is sometimes com- 

 paratively flattened (PL V. f. 3), or in the case of var. danubialis, 

 Servairi (PL V, f . 7 & 8) very swollen. 



This last-named variety was first pointed out from the Thames 

 Valley deposits by the late Dr. Boettger in correspondence. 



It is longer, narrower and very much more ventricose (ll'5x 

 8*8 X 8 mm.) in the adult stage, although rounder in the very young 

 stages than the type. Indeed the very young stages bear a close 

 resemblance to juvenile F. nitidum, and slightly older examples to 

 P. subtruncatum, in dentition as well as outline. It is less strongly 

 striate than the typical examples in the same bed, and much more 

 inequilateral owing to the greater truncation of the posterior end, 

 whilst the umbones, especially in the young shells, are more 

 rounded and prominent. The hinge-plate is also narrower, and 

 .there are minor differences in the dentition of the immature shells. 

 Suites are very easily picked out (PL VI, f. 3 ; VII, f. 3 & 4 ; IX, 

 f. 2, 5-7) and contrast well with similar series of the typical forms 

 from the same localities (PL VI, f . 1 ; VII, f. 1 & 2 ; IX, f. 3 & 4), 

 but linking forms are also met with (PL VI, f. 2). 



The only pearl I have seen in a Pisidium is in a specimen of 

 this variety from the Pleistocene deposits of Crayford-Erith 

 (PL IX, f. 7c). 



The trigonal variant above alluded to from the Pleistocene ot 

 Grays is probably the result of dwarfing brought about by un- 

 congenial environment, since it grades into the type (PL VI, f. 4). 

 The most pronounced specimens are cardiform in shape and 

 snbtriangular in lateral outline, with exceedingly heavy hinge 

 (PL IX, f. 9), suggesting at first sight very large examples of 

 P. supinum : two of these measure 7 X 6 x 5*4 and 6 x 5*5 x 4*6 mm. 

 respectively. 



The var. nova, Paul. (132, p. 176), is the form that in its adult 

 state departs most widely from the type. Both in external outline 

 and sculpturing (PL V, f. 17 d) and to a less extent in its hinge 

 (PL VIII, f. 2) it shows a curious approach to P. casertanum, but 

 in the younger shells (PL V, f. 17 a & b) the suggested resemblance 

 is much less strong, whilst the embryos which I obtained from an 

 individual kindly sent me by Herr Lindholm from the district of 

 Lake Baikal (as P. subtilestriatum, Lindh., 95, p. 84) are precisely 

 similar to those of the type form. 



c2 



