304 



TROCHID.E. 



with a rather narrow umbilicus, and is pure nacre ; the 

 other has a somewhat depressed spire and rounded 

 periphery, with a very wide and open umbilicus, and is 

 creamcolour with occasionally dark blotches. Possibly 

 these markings were caused by fossilization or mineral 

 action, and the prototype may have been as stainless as 

 its modern representative : 



" But no perfection is so absolute, 

 That some impurity doth not pollute." 



I am by no means prepared to assert that T. amabilis is 

 or is not a descendant of the fossil and so-called extinct 

 species, changed in the course of ages to a greater extent 

 than Terebratula caput-serpentis and other persistent 

 species ; our knowledge of such infinitesimally small or 

 differential gradations is at present too imperfect to 

 justify an assumption that " descent by modification" has 

 been the invariable or even the ordinary law of nature. It 

 would be inconvenient to retain the name (elegantulus) 

 which I once proposed for the present species, because 

 there is already a Trochus elegantulus, belonging to the 

 section Ziziphinus, as well as T. elegantissimus of the sec- 

 tion Margarita. A figure of the shell will be given in 

 the supplementary volume of plates. 



T. dnereuSf Couthouy (Margarita striata, Broderip and 

 Sowerby, but not T. striatus of Linne) has been dredged 

 by Mr. Waller on the Antrim coast, by Mr. Barlee in 

 Shetland, by Mr. Dawson in the Moray Firth, and by 

 Mr. Mennell in Berwick Bay ; but it is a submarine 

 fossil. It also occurs in the Clyde beds and at Uddevalla, 

 and inhabits the Norwegian and North American coasts. 

 This species differs from T. amabilis in its larger size, 

 greater solidity, dull grey colour, coarser and cancellated 

 sculpture, close-set and fine longitudinal striae, flattened 

 apex, and much smaller umbilicus. 



