236 Mr C. Collier on Univalves. 



from Heli.v, is a second. The single lip, caused by the con- 

 cealment of the columella from the roll of the body, is a third. 



There are individuals of Strombus, Murex, and Buccinum, 

 as strictly spiral as these, but their possessing the rostrum, a 

 more peculiar and therefore more distinguishing part, are 

 classed after it. 



Shells that are open, partially or completely, and of a flat 

 form, present, under such simplicity of construction, no feature 

 for association ; and hence a condition must be chosen. This, 

 perhaps, is a departure from the precision and perspicuity which 

 are the objects of all system, and make its chief value. But, 

 next to the perfect arrangements, based on essential organs, 

 must be reckoned that which is modelled from the assemblage 

 of the whole, or from the peculiarity such may present com- 

 pared with others. An essential condition is selected, when 

 objects are thus contrasted, and the line, if not drawn by Na- 

 ture herself, may be assumed as a circumscribing limit. This 

 is raised far above that which is drawn from relation to other 

 objects, and it is as much removed from opinion as such modi- 

 fication will allow. 



Some systems of modern date tolerate this license of compa- 

 rison, from the failure of discriminating features. The term 

 Pachydermata is taken from a relation of one part only, and 

 a relation which, literally considered, is not universally true. 

 Rapaces, among birds, is from a mere quality, which, exposed 

 as it is to opinion and opportunity of observation, is only a con- 

 ventional term, and obviously an artificial bond of union. 



The shells, then, of the Linnaean Haliotis and Patella, unit- 

 ed by a distinguishing condition (apertum), may be formed 

 into sub-genera by the presence or absence of the only secon- 

 dary qualities which belong to them, namely, the presence or 

 absence of a margin. 



7. Although the inhabitants of tubular shells are not, on the 

 authority of M. Cuvier, constituted like those of Testacea, yet, 

 as calcareous productions, they have been considered within the 

 province of the conchologist. This shape, being the common 

 feature, forms the family, and unites, of course, Serpula, Den- 

 talium, Sabella, and Teredo. The divisions arise from pecu- 

 liarities which well enough designate the individuals, and may 



