4 CATALOGTE OF 



In common with most freshwater mollusca the various species 

 of Pisidium are extremely susceptible to the influences of their 

 environment, and thus vary greatly in their external appearance 

 according to their habitat. 



If instead of confining one's attention to the study of rows of 

 cabinet specimens from various localities, each too frequently 

 consisting of only some three specimens, the mixed gatherings of 

 species from different places be taken, it speedily becomes evident 

 how great is this effect of environment. In one locality, all the 

 species will exhibit less sculpturing than the normal forms ; in 

 another, more ; all may be dwarfed, or all abnormally large ; 

 occasionally one species in the gathering will show greater increase 

 in size over the average, while its associates are undersized ; or 

 exaggerated inilation may be the prevailing feature. Thickening, 

 or attenuation of the shell, or abnormalities, may likewise prove 

 characteristic of certain localities. 



As Jeffreys remarks (and similar observations were made by 

 Jenyns and others): "Size, substance, sculpture, and lustre are 

 not of much account, as they mainly depend on the chemical 

 ingredients of the water inhabited by these mollusks, as well as on 

 their supply of food" (81, vol. i, p. 18). 



In the course of the present investigations it was found that 

 each species varied in outline from what came to impress itself as 

 the normal form, towards a more rounded shape on the one hand 

 and a more elongate on the other, as well as to assume sometimes 

 a more swollen, sometimes a more flattened condition ; whilst the 

 hinges of those taken in quiet waters are feeble and weak, in 

 comparison with the hinges of those habitually dwelling in running 

 water. 



Often, while it is comparatively easy to separate the individual 

 species in a given gathering (A, B, and C from locality X, or 

 locality Y), it is a matter of trouble to discriminate between 

 samples of different species from diverse localities (B from locality 

 X may be more like C from locality Y, and so on), if only the 

 external characteristics he depended on. When, moreover, col- 

 lections from the numerous post-tertiary deposits are examined, 

 the difficulty is enhanced, for then, swept together by the flood- 

 waters in which the deposits were formed, one meets with the 

 mixed rejectamenta of a wide area, where varying environments 

 abounded, whilst the amount of wear and tear the specimens have 

 frequently undergone does not render the task of their identification 

 any the more easy. 



The question arises, what then are the best characters to rely 

 on in distinguishing the species. Jeffreys (81, vol. i, p. 18) 

 recognized that little reliance could be placed on the soft parts 

 or at all events on their external features. Nor would these avail 

 in the present investigations in which it is sought to deal equally 

 with fossil forms. The external form of the shell, on which 

 Jeffreys somewhat reluctantly depended, we have seen to be 

 unreliable. Fortunately the Pelecypod shell far more closely 



