Jenyns's Monograph on C^clas and Pisidium, 375 



not clearly defined," &c. The author has endeavoured, and 

 successfully it may be said, to amend PseifFer's errors and 

 supply his defects ; and states, that, in so doing, he has, as to 

 the relation of the parts of the anhnals and their shells, and 

 the terms used to designate them, adopted the views of 

 Blainville, as presented in his Manuel de Malacologie. 



Of Cyclas, three species are described, viz. x rivicola Leach, 

 cornea Lamarck, and calyculata of Draparnaud ; as well as 

 varieties of tliese. Of Pisidium, the species presented are, 

 obtusale Pseiffer, with varieties; pusillum Jenyns, with va- 

 rieties; nitidum Jenyns ; pulchellum Jenyns, with varieties; 

 Henslowm?zw?7? Jenyns ; and amnicum Jenyns, with varieties. 



Synonymy is ever a matter of much importance to sys- 

 tematic naturalists ; and this monograph will to such be really 

 valuable, for the diligence with which synonymes have been 

 collected from Continental as well as from British writers, 

 and for the cautious, judicious, and therefore instructive, ad- 

 justment of them. The following quotation will evince the 

 caution exercised : — "In my attempt to determine the species, 

 i have not merely considered the general characters of the 

 shell, but have derived much assistance from attending to 

 the animal inhabitant. Indeed, it is absolutely requisite to 

 caution conchologists against drawing any conclusions, with 

 respect to the specific distinction of these animals, from a 

 mere inspection of the shell alone. This is so liabl6 to vary, 

 from age, peculiarity of situation, and probably from other 

 causes, that it becomes necessary, in some cases, to compare 

 Si large number of specimens, collected from diffb'ent sources, 

 in order to determine the characters of a single species with 

 ^ny degree of precision. Occasionally the shell becomes 

 exceedingly ventricose, at the expense of its height, which is 

 thereby considerably diminished; and the valves, which 

 perhaps naturally meet at an acute angle, under such cir- 

 cumstances meet at an obtuse one. Neither can sculpture 

 be relied upon, the striae (furrows) varying exceedingly in 

 number and distinctness, according to the nature of the water 

 in which the shell is found : a circumstance of which Dr. 

 Leach was not sufficiently aware when he formed three 

 species out of Pisidium amnicum. Age, likewise, produces 

 great changes : not only are young shells much more com- 

 pressed than adult ones, but in many instances the relative 

 proportion of their parts is different. Indeed, in the case 

 •of the minute species, so great and general a similarity pre- 

 vails amongst their young, that it is hardly possible to identify 

 them in this state without the closest examination." From 

 .the above remarks and quotation it will be apparent that the 



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