Terrestrial, Fluviatile, and Marine Shells. 161 



ter, M'hich is covei'ed by a surface having the form of a shell, 

 afforded distinct traces of crystalline facets, some of which ap- 

 peared to he triangular like those of the diedral terminations of 

 arragonite, and others in the form of elongated parallelopipeds 

 like the faces of the prism of the same mineral. I coiUd not re- 

 duce these faces to the rhombohedral system of the genus cal- 

 careous spar; and thus, although I could not determine the form 

 of these ci'ystals, owing to their being so confusedly grouped to- 

 gether, this circumstance, in addition to the complete absence 

 of lamellar texture, a slightly resinous though pretty high 

 lustre, and an appearance perfectly analogous to that of ar- 

 ragonite, appeared to me entirely to distinguish this crystal- 

 line mass from calcai'eous spar. Besides, it makes a deep 

 scratch on the clear and crystallized Iceland-spar. 



I afterwards remarked that the shells of the Helix poina- 

 tia, the Anadonta anatitia, and the Unio pictorum, likewise 

 scratched the Iceland-spar. All of them, as well as the Li- 

 macella, effervesce briskly with nitric acid. The Anadonta 

 has two layers of nearly equal thickness, the upper composed 

 of crystalline prisms Avith axes parallel to each other and 

 perpendicular to the plane of the layer, the lower of compact 

 mother of pearl. In the Unio pictorum, the upper layer is very 

 thin, and the mother of peai'l very thick. 



I now give the list of all the shells which I have tried, and 

 which scratch the crystallized calcareous spur more or less 

 strongly. 



Terreslritd and FluciatUe Shells. 

 Liiuacdla, (strougly). Helix pomatta, (rather strongly), llel'x nemoralis, 

 yellow, adult, having a peifect mouth, (strongly). Helli- nemomlis, yellow, 

 young, with an imperfect mouth, (feebly). Ildiv oaihusumeUu, living shell, 

 with the mouth, (very feebly on account of its slight thickness and its great 

 fragility). Helix ericetorum, {vaiher strongly). Physa fontinalis, becomes a- 

 braded at the thinnest side of tlie moutli, but it scratches deeply, although, 

 on account of its fragility, much pressure cannot be employed. Lpunwus 

 auricuiiris, (scratches, although it is fragile). Li/MiicuKsstagnalis, (scratches 

 strongly, although fragile). Anadonta anntliia (sti-ongly). Anadonta cygmea, 

 (i-ather strongly), llnio jnctorum, (strongly). Cydas riealig, dead and alter- 

 ed, (scratclies deej)ly, l)ut becomes itself abraded). 



Marine Shells. 



Oftna eddUs, (scratches very strongly). Osliea parasitica, (still more so), 

 Anumiu fphippiiim, (feebly). Anoniia cyiindricn, (very feebly, on account of 

 VOL. XXVII. NO, LIII. Jfl-Y 1831*. I. 



