SPECIES OF riSJUlUM. 11 



1. P. pusillum (including 4. P. Itensloiviaiium (including 



obtusale). pulcheUtim and cinereum). 



2. P. nitidum. 5. P. amnicum. 



3. P. roseum. 



When it is considered that examples of P. subtruncatum 

 were intermixed with Jenyns 1 P. lienslowianum and P. pul- 

 chdlum, and that fine specimens of P. subtnincatum do bear 

 some external resemblance to P. cinereum, Jeffreys' No. 4 is 

 not so incomprehensible ; hut what is to be said of his 

 association of two such distinct forms as P. pusillum and 

 P. obtusale ? 



One feasible explanation appears to be that Jeffreys 

 unconsciously transposed Jenyns 7 P. obtusale and P. nitidum, 

 just as on the two occasions in which in his '* British 

 Conchology " (vol. i, pp. 32 & 38) he described the teeth 

 of Unio, he in each case transposed the two valves and 

 described right for left and vice versa. 



This hypothesis receives support from the discovery in 

 the T. Rogers collection of Pisidia (now in the writer's 

 possession) of two tubes marked " Pisidium nitidum from 

 J. G. Jeffreys " and " Pisidium nitidum, var. globosn, 

 J. G. Jeffr. (new)" (PI. XXX, f. 5), both containing speci- 

 mens indistinguishable from Jenyns' type specimen of 

 P. obtusale at Bath. Furthermore, in " British Conchology " 

 P. nitidum is classed by itself under the section " C. 

 Round," which is exactly what Jenyns' P. obtusale is and 

 his P. nitidum is not. At the same time the confusion, if 

 such it be, is made worse by his retention of the descriptions 

 of some of the characteristic features as given by Jenyns 

 under the latter's names. On the other hand, ovate forms 

 of P. nitidum are externally very like some fojms of 

 P. pusillum, as Jenyns especially pointed out in his notes 

 to which allusion has just been made, while there is little 

 doubt, though it was not possible to prove the fact by 

 opening them, undetected specimens of P. personatum were 

 intermixed with both.* 



Jeffreys, however, did recognize the distinctness of Jenyns' 

 P. pulchellum, var. c and its identity with P. yassiessianum 

 of Dupuy, but was less happy in attributing it to P. roseum, 

 Scholtz (PI. XIII, f. 9 ; XV, f. 11), solely on the supposed 

 identity of the colour of the animals, when he had never 

 seen Scholtz' specimens ; still, since the name of P. niilium, 

 Held, has the prior claim, no more need be said. One 

 other fact of importance should be noted, namely, that 

 at this period, Jeffreys spoke of "P. fontinale of I 3 feiffer 

 (our P. pusillum) " (p. 38), and in Rogers' collection there 



* Of two scries of specimens in tbe Norman collection at the British 

 Museum, marked 'Pisidium nitidum. Examined by Jeffreys," one 

 [I1M1.10.26: 7775-1)3] was P. personatum and the other [1911.10.26; 



7M )( > -}. r >] P. j'HfillicHi, immature and rather rounder than usual. 



