XCVi INTRODUCTION. [CH. 



luscous shells entitle his opinion to the greatest possible 

 weight ; and he professed that he was unable to detect 

 any apparent difference between the texture of these 

 specimens and of others (which were unquestionably 

 recent) belonging to the same species and placed with 

 them for the sake of comparison. No chemical or other 

 test seems to be known, by which the texture of shells 

 called fossil, and certainly of very remote antiquity, can 

 be distinguished from that of recent shells. The gloss 

 and the greater or less transparency of the latter, con- 

 trasted with the dull aspect and opacity of the former, 

 afford the only criteria of distinction; but it is not 

 known how far the continued submersion of shells for 

 many ages in the sea, where they are placed beyond the 

 reach of atmospheric influence, may have prevented any 

 change in their external appearance. The shells of Mol- 

 lusca would seem to be nearly indestructible by the 

 ordinary action of air and water, and especially when 

 their structure is crystalline and compact. The term 

 " recent " is, of course, comparative in point of time. But 

 a few of the shells from the Turbot bank, belonging as 

 well to some of the species in question as to other species 

 which are undoubtedly indigenous and exist there in a 

 living state, have every sign of being fossil, and are pre- 

 cisely similar in appearance to the shells which are found 

 in the Clyde and other beds of a pleistocene formation. 

 Some of these beds occur in the neighbourhood of the 

 Turbot bank, and contain Yoldia lanceolata, Leda pyg- 

 maxi, Hypothyris psittacea, and other shells of a decidedly 

 Arctic character ; but only one of these species (viz. Leda 

 pygmcea) has been observed in theTurbot-bank dredgings, 

 and of this species Mr. Waller found a living specimen. 

 Columbella Holbollii, Scalaria (?) Eschrichti, and Mar- 

 garita cinerea (being three out of the five Arctic species^ 



