274 CARDIIDiE. 



the posterior extremity, showing that the same kind of 

 monstrosity occurs in different species which agree in 

 general structure. 



In the present instance I have omitted some of the 

 characters, believing that a comparison between this 

 species and the last would be sufficient. The two are 

 very similar. Such a strong family likeness runs through 

 all the Cockles, and indeed through every other equally 

 homogeneous group, that some may think it superfluous 

 to repeat the same particulars in describing each species. 

 But nothing is more difficult, and often more unsatis- 

 factory, than to compress the description within narrow 

 limits. In attempting to seize the salient points, others 

 which perhaps are not less material are liable to remain 

 unnoticed. " Dum brevis esse laboro, obscurus no." 

 More harm to science has been done by a slight and 

 consequently vague diagnosis than by a too great pro- 

 lixity of detail. 



This species may be the C. rusticum of Linne; but 

 there is equal probability that it is his C. tuberculatum, 

 which is of prior date. The latter name only is given 

 in his descriptive Catalogue of the Museum of Queen 

 Louisa Ulrica ; and the comparison there made between 

 the present species and C. echinatum is full and con- 

 clusive. Even in the ( System a Naturse/ which contains 

 a description of all three species, C. tuberculatum is 

 placed next but one to C. echinatum, and C. rusticum 

 follows C. edule, which is separated from the first by 

 eight other species. I have observed a similar order of 

 sequence in the present work, as I consider it a great 

 help to classification, and that this method of asso- 

 ciating or bringing together species closely allied to 

 each other enables the student to distinguish them 

 more readily than if they were arranged indiscrimi- 



