OF CONCHOLOGY. 171 



examining his original microscopic slides and the photograph 

 taken from them. No such teeth as are indicated for the first 

 five Literals in this figure, are to be found on the original radula. 

 The central tooth is almost certainly bidentate instead of simple. 

 The specimens referred to, now in the Mus. Comp. Zoology, 

 were unfortunately mounted in Canada balsam, which, by ren- 

 dering the teeth quite transparent, ruins them as objects of 

 reference. Still, enough was made out to render the above state- 

 ments positive, and they were still further confirmed by the 

 radula of the fresh specimen, although the latter was somewhat 

 immature. If further doubt existed as to the erroneous nature 

 of the figure a careful examination of the photograph, taken from 

 the original radula before it was spoiled by the balsam, would 

 satisfy the most incredulous. An examination of twenty-five 

 radulse of a closely allied species of the same genus, disclosed 

 the fact that the outer laterals of effusa are furnished with one 

 or two more denticles, while the inner laterals appear nearly 

 identical. The error was almost certainly caused by mistaking 

 and confusing the outlines of the bases as seen through the trans- 

 lucent cusps, with the outlines of the cusps themselves. 



We have not at hand a specimen of Planorhis trivolvis to com- 

 pare with fig. 10. Any of our readers who possess a first class 

 microscope will be able to make the necessary comparisons dur- 

 ing the coming season. The teeth of J*, lentus, as we have ex- 

 amined them, difter considei'ably from those of P. trivolvis as 

 here figured. The cusp of the rhachldian tooth appears to us to 

 be bifid, rather than furnished with two spike-shaped denticles. 

 The teeth of Tidotoma magnifica, long a desideratum, are so 

 simple that it would be difficult to fall into serious error in figur- 

 ing them. It may, however, well be doubted if the denticula- 

 tions are so ragged and irregular as here figured, and indeed it 

 would be satisfactory to have any of the figures in this paper 

 confirmed by more careful drawings. 



In conclusion-, we would remark, that any practical microsco- 

 pist will feel the utter inadequacy of the process previously de- 

 scribed, for furnishing reliable figures of anything, unless checked 

 and controlled by a trained eye, experience with the microscope 

 and with the special department under consideration. That pho- 

 tography can greatly assist there is no doubt, but as a sole re- 

 liance it is untrustworthy. 



We feel sure that the gentlemen Avhose names head this review 

 will regret as much as we do that their wide and well deserved 

 reputation should give Aveight and authority to such errors, and 

 that they will consider this review to be, as it is, Avholly prompted 

 by the interests of Science, and totally free from any personal 

 considerations. 



