386 VENERIDiE. 



1835, Mr. Clark had carefully investigated this point, and 

 his observations, which on account of their value, we quote 

 at full, appear to us decisive. " Having dissected," writes 

 that most careful observer, " examined, and compared 

 many of the animals of these hitherto- considered distinct 

 species, I have found that in every respect they agree with 

 each other, and that there is not even the slightest variation 

 in the conformation of any of their organs either external or 

 internal. The branchiae of the two, in colour and form, are 

 exactly the same. The foot has precisely the same slit in 

 its centre for the byssal filaments, and we have seen the one 

 imbedded in its cavity attached by them, and the other 

 naked, from the shingle attached in like manner to particu- 

 lar stones. The margins of the mantle are the same in 

 both. The tubes extend, bifurcate, and vary in their rays 

 and ciliations in the same degree in both. In short, we 

 cannot point out any particular organ but what is exactly 

 similar. We are therefore compelled by the force of facts 

 to come to the conclusion that the two are identical ; and 

 though the shells of each seem to present greater differ- 

 ences than their animals, still the principal form and out- 

 line of both, with their strise of growth and decussations, 

 are essentially of the same character ; the variations are 

 mere modifications of similar elements dependent on habi- 

 tat, deprivation of light, or exposure to it, for when the 

 shells are embedded in the cavities of rocks they are sub- 

 ject to variations of form, and being deprived of light, they 

 are usually colourless and without markings ; and not 

 being rubbed by the action of the waves amongst the 

 shingly beaches, their stria) are sharper and more foliaceous. 

 Amongst shingle, where they are free, or if the current be 

 strong, attached by their filaments to stones, and exposed 

 to light and the action of the sea, they are of regular form. 



