THE GENERAL SUBJECT. 
113 
himself. These observations are made chiefly on discoidal 
shells {Ammonites j Planorbis), or others with a short blunt spire, 
as, for instance, Naiica glaucina and aperta. Shells with an 
elongated pointed spire offer much more difficulty. 
Cf. also Observations on the mode of growth of discoid and 
turbinated shells,^^ by A. Macalister, Ann. N. H. (4) vi. 1870, 
p. 260, and P. 11. Soc. xviii. 1870, p. 529 (omitted from preceding 
vols. of Zool. Rec.). 
The choinical composition of the shell of 7 species of ruhnofmtn and 
4 of freshwater bivalves has been analyzed by A. Boring, Inaugural Bisserta- 
tion, Gottingen, 1872. 
S. Clessin describes the habits of snails during the winter, their burying 
in the ground, often in crowds, the formation of the epiphragm, the inter- 
ruption in the growth of the shell &c. He thinks that slugs and freshwater 
snails are less sensible to the influence of season, hiding themselves later in 
autumn, and coming forth earlier in spring than Helix, and that young speci- 
mens are less sensible than older ones. CB. Ver. Regensb. xxvi. pp. 114- 
130. 
A specimen of Helix pomatia lived for 11 months without feeding, and 
slept for 17 weeks. Its weight was diminished by 043 gr., or O'O per cent, 
daily. J. v. Sivers, CB. Ver. Riga, xix. p. 112. 
The growth of Limnoia stagnalis is much slower if young individuals 
are confined in society than when isolated, although apparently enough 
food is provided. C. Semper (Wiirz. nat. Z. ?), separate print of 9 pp., 
1872. 
Oysters kept in water with ]'8-B2 per cent, salt lived for some time, but 
became very thin. Meyer, Nachr. malak. Gcs. 1872, pp. 3-5. 
Momtrosities and Deformities. 
A rather large number of scaliform specimens of Planorbis 
complanatus {marginatus, Drap.) have been observed by L. Pire 
in a pond thickly covered by Lemna minor. Van n. Broeck 
suggests that the difficulty encountered by flat-shelled mollusks 
in making their way through the thick layer of duckweed may 
cause these deviations (Bull. klal. Belg. vii. pp. x-xxi), and 
proves by experiment that scaliform specimens make their way 
more easily through the duckweed than those of normal shape. 
Ibid. pp. xxxiii-xxxviii. Cf. also a paper by Pire, Brussells, 
1871, 7 pp., 2 pis. 
S. Clessin, discussing similar cases of deformities in 
fontanus, albus, deformis, and contortus, in which (either in a pond 
filled with water-plants or on the shores of lakes between stones) 
many individuals exhibit distorted or screw-like separated whorls, 
insists that there can be no transmission of these deformities, but 
that each individual, when growing, is caused to deviate from the 
normal form bv obstacles or external agencies. Mai. Blatt. xx. 
pp. 68-83. 
