454 Dr. Wallich on the RMzopods. 



specieSj in likeness with the species of higher organic forms, 

 more or less intimately related, and by intermediate forms 

 or varieties merging into one another." 



Having got rid of every question connected with the cha- 

 racters of the animal, it is, at all events, satisfactory to feel 

 that the present inquiry is reduced to the, comparatively 

 speaking, narrow issue of determining whether the characters 

 of the test in Quadrula are, or are not, sufficiently unique and 

 constant to warrant the creation of a new genus for the recep- 

 tion of Diffiugia symmetrica. 



As we have already been made acquainted with the cha- 

 racters of ''^Quadrula'''' symmetrica as laid down in Prof. 

 Leidy's work^ let us now turn our attention to the characters 

 therein given of the genus Difflugia, bearing in recollection, 

 however, that a genus is little better than an abstract concep- 

 tion, and that it behoves us to assure ourselves, at every step of 

 the investigation, that the characters assigned to this particular 

 genus as a whole, neither exceed nor fall short of the aggregate 

 characters of the several species included in it. It will be 

 understood as we proceed why this caution is considered 

 necessary in dealing not only with the new genus Quadrula, 

 but with the equally questionable new genus Nebela. 



And here let me observe that, owing to the unusual length 

 and vagueness of many of Prof. Leidy's definitions and de- 

 scriptions, however desirable it may be to epitomize them to 

 the utmost, the end in view would be defeated were any 

 passages struck out which the author might consider essential 

 to the due comprehension of the points at issue. When it is 

 stated that thirty-two pages of letterpress are devoted to the 

 genus Difflugia as a whole, seven to D. pyriformis, and the re- 

 maining twenty-five to D. globulosa^ 1). urceolata^ D. cratera, 

 D. acuminata, D. lohostoma, D.arcula^D. corona, D. constricta, 

 and, lastly, D. spiralis (of which at least half are, on Prof. 

 Leidy's own showing, most variable species), the difficulties 

 in the way of condensation beyond a certain point will, I 

 venture to think, become apparent. 



The characters given of the genus Diffiugia are : — " Shell 

 very variable in shape, usually composed of extraneous angular 

 particles of hyaline quartz-sand, sometimes mingled with other 

 bodies, such as diatom-cases, sponge-spicules, &c. ; the same 

 forms sometimes composed of chitinoid membrane incorporated 

 with scattered extraneous particles, or composed in part or 

 entirely of intrinsic particles of peculiar character. Mouth 



inferior, usually terminal Pseudopods usually up 



to half a dozen or more, cylindrical, simple or branching, com- 

 monly round at the ends, sometimes spreading and pointed." 

 — Op. cit. pp. 95, 96. 



