SPECIES OF PISIDIUM. 9 



dropped. His description suggests one of the forms of 

 P. pusillum. Jenyns assented to the determination of the 

 second species, but Macgillivray says "P. jenynsii and 

 P. pidclidlum .... and all their varieties, I am almost 

 persuaded, are of one single species" (p. 251). 



Macgillivray'a third species, if it was identical with 

 Jenyns' specimen, would be P. subtruncatum. 



1849. Forbes and Hanley published their classical work (63), and, 



when treating of Pitidiwn, followed Jenyns, adding, of 

 course, P. cinereum, Alder. They cite Jenyns as having 

 reverted to the integrity of his P. puklidlum, but at, the 

 same time shrewdly suggest that his var. c) " appears the 

 least connected with the rest." (Vol. ii, p. 129.) 



1850. Schmidt (156) defined P. supinum, but it was some years 

 before his species was recognized. 



1852. The section of Dupuy's celebrated "Histoire" (55) was 

 published containing the chapters on Pisidium. He dis- 

 tinguished 12 species, five of which have passed into 

 synonymy, the only item now of interest being that he 

 ranked Jenyns' P. pusillum as a synonym of his P.fonli- 

 iiale. 



1855. Malm (106), dealing with the Swedish non-marine Mollusca, 

 enumerated nine species, of which two, P. sultruncatum 

 and P. personatum, were new and of great importance, 

 inasmuch as it was failure to recognize these two forms 

 that led to much of the confusion into which the work of 

 British and other authors had been thrown. One of Malm's 

 species, which he believed to be new, P. arcceforme, is 

 identical with P. milium. 



1854. liourguignat published a note on Pisidia (21), in which he 



laid down the utility of the hinge as a basis for classifica- 

 tion. His species were those of Jenyns, with the addition 

 of P. casertanum (which, however, was made to include 

 pulcliellum and the forms now referred to milium) and a new 

 species P. redutianum, which Jeffreys considered (80, p. 39) 

 to be founded " on young specimens of P. hensloiviamtm 

 (var. pulchdlum)" but which rather suggest fry of 

 P. amnicum. 



1855. Gassies, in his description of the Pisidia of Aquitaine (65), 

 raised the number of species to fourteen. Four of these 

 he supposed to be new, but they have passed into synonymy. 



1856. Moquin-Tandon in his well-known " Histoire " (116) 



acknowledged only six species : 



1. P. liensloiuanum. 4. P. nilidum. 



2. P. amnicum. 5. P. pusillum. 



3. P. casertanum. 6. P. obtusale. 



He included Jenyns' P. pulchdlum, and consequently 

 P. mtiiwH) under P. uuserUinum. 



