310 APPENDIX. 



This is not very rare at Clacton ; at Copford, Mr. Brown tells me, it is so. It closely resembles //. 

 rotundatd in its large umbilicus and elevated ridges, but is justly separated from that species by well-marked 

 characters, having large volutions, which are fewer in number, and the aperture is consequently more open. 

 U.slriatella, Autliony, 'IJost. Jouru. Nat. Hist.,' vol. iii, p. 278, pi. 3, fig. 2, an existing species in the United 

 States, is now admitted to be the same as the above ; and //. perspectiva i.s, perhaps, only a variety. A 

 species, with a geographical range extending from Russia and Switzerland to Massachusets and Ohio, 

 might be expected to possess considerable variation.* In my List of Land and Fresh-water Shells from the 

 Older Tertiaries, at Ilordwell, published in the 'Lond. Geol. Journ.,' vol. i, p. 1 18, I had considered a species 

 found there to have prolonged its existence tmckanged to the present time, and that the one above referred 

 to (striatella) was its specific descendant. Mr. F. Edwards, in his 'Monograph of the Eocene Moll. 

 Pulmonata,' p. 66, gives his opinion adverse to that decision ; and I am now inclined to think be is correct. 

 //. lahyrinthica was given in my list as one of the species existing at the present time, while two others 

 were also inserted there, but with a doubt. I was at one time impressed with the idea that an air-breathing 

 mollusc, being less exposed to a variation of conditions, was enabled to preserve a specific existence longer 

 than its marine brethren ; and I fear it gave a slight bias to my determinations. Mr. Edwards has bestowed 

 upon the lalijrinthica a very critical and a very able examination, and has pointed out some differences 

 between the Hordwell fossil and the existing American shell ; but they were not, he considered, sufficient to 

 invalidate their identity, more especially as the living shell is itself subject to considerable variation ; 

 assuming, also, that if we possessed a large series of fossil specimens, these differences would probably 

 disappear : is it really certain that the possession of a larger series must necessarily bring the recent and 

 fossil nearer together? This species of mollusc appears to stand almost, if not entirely, by itself, as dating 

 its existence from the Older Tertiaries ; and, as wc are obliged to depend exclusively upon the restricted 

 characters of the shell alone, we are compelled to admit its identity, from the impossibility of pointing out a 

 dissimilarity such as is generally considered sufficient for specific separation. Might it not be possible 

 that the combined characters of the entire animal, rould they be possessed, would exhibit a difterence so as 

 to prevent its coming within the definition of a species in the present acceptation of that term? If this 

 Hordwell fossil be really still an existing shell, it militates somewhat against the hypothesis that a species 

 with a very prolonged existence in regard to time, would, if still in being, have a more extended geo- 

 graphical range, and vice versa ; it does not appear so in regard to Helix, when labyrinthica is compared 

 with rtulerata. 



SIPIIONOSTOMATA. 

 MiTHA EBENCS, LamarcJi. Tab. XXXI, fig. /. 



MiTR.v EBENUS. Lam. Ann. du Mus., vol. xvii, p. 216, No. 58, 1822. 



— — Fhil. En. Moll. Sic, vol. i, p. 229, t. 12, figs. 8— 10. 



— — Bellardi. Monog. delle Mitre Foss. del Piedmonti, p. 23, t. 11, figs. 



20—23, 18.)0. 



— PYRAMIDELL.\. Bronn. var. p. Ind. Palrconth., p. 733, 1848. 



— PLUMBEA. Laink. Hist, des An. s. Vert., tom. vii. No. 73, 1822. 



* This is the only one of the six extinct species belonging to these Fluviatile deposits that has us range 

 to the westward ; the others belong to the European Continent, with the exception of Cyrena consolrina, now 

 an inhabitant of the Nile. The MoUusca of these beds do not appear to have any connection with the 

 peculiar, and probably more recent, Faunas of the Canaries, Madeiras, or the iVzores. 



