SAXICAVA. 137 



When we compare the animal of Saxicava with that of 

 Gastrochtena, we are at once struck with the near resem- 

 blance and evident affinity of the two genera, so near, indeed, 

 that it is difficult to draw a well-marked line between them. 

 We find the same structure of mantle, the same form of 

 foot, and very similar siphons in each. The shells are also 

 very similar, and some of the varieties of Saxicava rugosa 

 are so like in every respect to Gastrocliana^ that, did we not 

 know their history, it would be difficult, if not impossible, 

 to pronounce on their generic position. But when we attend 

 to the changes which the former shell presents in the course 

 of its growth, and compare the several congeneric species at 

 different ages, we recognise a marked source of distinction, 

 which, combined with the habit in Saxicava of never forming 

 a tube, whilst Gastrochana always, if possible, makes one, 

 warrants the separation of the two genera. Those naturalists 

 who have either not been acquainted with, or have not un- 

 derstood the animal of Saxicava have been extremely 

 puzzled as to its true position, whilst the variable characters 

 of the hinges of our European species have increased the 

 difficulty and led to the institution of several spurious 

 genera. The generic terms Byssomya^ Hlaiellai Rhom- 

 boides, Biapholius, and Agina, have not been proposed as 

 mere synonyms of Saxicava, but as so many allied genera 

 of equal rank, all constituted out of the typical species 

 of the genus ; whilst Mya, Solen, Donax, Mytilus, and 

 Anatina, have at various times, and some simultaneously, 

 numbered among their spurious adherents varieties of the 

 same Protean shell. Its true position has been equally 

 misunderstood, its close affinity with Gastrocha;na having 

 rarely been recognised : more usually it has been placed 

 between Solen and il/ya, a false position assigned to it 

 even by Cuvier. This is the more remarkable, seeing that 



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