132 



NATURAL SCIENCE. 



August, 



whose relationships are therefore less likely to have suffered complete 

 obliteration." The Devonian and Carboniferous periods are not 

 usually treated with such irreverence. But, apart from that, the 

 argument is curious : if all molluscs arose first in the sea, surely those 

 modified for land and fresh- water conditions must have their origin 

 more obscured than molluscs that remain marine. " Snails as 

 Barometers " is the heading of a paragraph on p. 50 ; but the subject 

 thereof is snails as hygrometers. A short time ago there was a note 

 in Natural Science on Xenophora (Fig. 2) ; Mr. Cooke has an idea that 

 when this genus coats its shell with foreign objects, " sometimes the 

 selection is made with remarkable care" (p. 64); one would like 

 evidence that the apparent selection is due to anything else than the 

 nature of the surrounding objects. As instances of molluscs that can 

 leap away from their enemies, Mr. Cooke, on p. 65, mentions the 

 Strombidse, Terehcllnm, and Trigonia ; he might have added the 

 common Pecten, with which one can experiment any day. The 

 following statement on the same page seems a trifle imaginative : 

 " Many Cephalopoda emit a cloud of inky fluid, which is of a somewhat 

 viscous nature, and perhaps, besides being a means of covering retreat, 

 serves to entangle or impede the pursuer." " The Mollusca," it is 

 said on p. 67, " are not much mimicked by creatures of different 

 organisation " ; to the few instances here given one may add the 



Fig. 3. — Three stages in 



the growth of Cypraea exanthema ; from specimens taken at 

 Panama. 



curious resemblance of the annehd Autodetus to the gastropod Proi'o- 

 calyptraa, both in the Devonian rocks of New York {see J. M. Clarke, 

 Avier. GeoL, xiii., p. 327). In his cases of commensahsm Mr. Cooke 

 might include the strange story of the worm, the gastropod, the coral, 

 and the bivalve, related by Bouvier (Nat. Sci., vol. v., p. 86). The 

 section on Variation is valuable ; this and the numerous figures of 

 stages of growth may possibly help to check the rash coining of new 

 specific and varietal names (Fig. 3). But we fail to see why specific 

 differences should be confined to '■'structural difference in the organisation 

 of the animal (as distinct from that of the shell alone)." Surely the shell 

 is as much a part of the animal as, say, the liver. The slight calcifi- 

 cation of Mya arcnaria and Tellina halthica in the eastern parts of the 

 Baltic is ascribed to " the violent changes of temperature which are 

 experienced in the Baltic " (p. 84). Might not the greater freshness 

 of the water in that part be a still more potent cause, just as it is in 

 the case of Littorina (p. 93) ? On p. 137, speaking of the dibranchiate 

 cephalopods, the author says, " It is not yet known how the 

 spermatophores find their way into the hectocotylus, or how the 

 hectocotylus impregnates the ova of the female " ; thanks to Racovitza 

 this statement is no longer true {see Nat. Sci., vol v., p. 321). On 

 p. 400 we are told how fast Chiton can walk, and how it rolls up like 



