342 Dr. J. E. Gray on Seals. 



Gastropods, on the under surface of the water, with its shell 

 downwards. 



Inhabits the Bolton and Gorton Canals at Manchester. 



Suspecting that this American species had been introduced 

 into our canals through the cotton-mills, I wrote to Mr. Rogers 

 for information ; and he tells me that in one habitat (and pro- 

 bably in the other also) the waste from the first process or 

 " blowing-machine " is discharged close to that part of the 

 canal where the Planorhis occurs. As the best cotton is culti- 

 vated in river-bottoms, and the crop, when picked, is spread 

 out and dried, nothing is more likely than that it should take 

 up either the Planorhis or its eggs ; and these could be trans- 

 ported alive to any distance. The vitality of Planorhis ^ and 

 its capability of enduring considerable changes of temperature, 

 may be inferred from the habit which certain species are known 

 to possess of closing the mouth of the shell in summer (when 

 the shallow pieces of water in which they live are dried up) 

 with an epiphragm or membranous lid, to exclude the heat 

 and prevent the evaporation of the natural moisture. Thus 

 protected, they keep alive for weeks, and even months, until 

 the return of the rainy season. 



In connexion with the foregoing, I would suggest that 

 Spluerium ovale may have been introduced in the same or 

 some other way from the United States. That species also 

 inhabits the canals near Manchester, and may be the Cyclas 

 transversa of Say. It has long been known in this country. 

 I have a specimen which was iu Dr. Turton's collection of 

 British shells more than forty years ago. 



I have written to Mr. Anthony, of Cambridge, Mass.^ one 

 of the leading conchologists in the United States, for informa- 

 tion as to the range of distribution there of both these species, 

 and especially as to whether they, or either of them, inhabit the 

 cotton-growing districts. 



Several species of land-shells (e. g. Zonites cellarius and 

 Helix nemoralis^ var. horte?isis), and perhaps of freshwater 

 shells also, are supposed to have been introduced into North 

 America from Europe by the agency of man, and are now 

 thoroughly acclimatized in the former continent. 



XLI. — Notes on Seals (Phocidae) and the Changes in the Form 

 of their Lower Jaw during Orowth. By Dr. J. E. Gray, 

 F.E.S. &c. 



One of the most important studies of zoologists has been the 

 examination and comparison of the differences in the colour 



