408 Mr. W. Clark on British Mollusca. 



then applied to Mr. Jeffreys for an example : this was forwarded ; 

 and after an interchange of the minute shells in question, and having 

 opened Mr. Jeffreys's example, and compared the dentition, muscular 

 and pallial impressions, and absence of siphonal scar, with mine, I 

 instantly perceived that my young Diplodonta rotimdata and his 

 young shells of the supposed Biodonta Barleei were identical. I 

 pointed out to Mr. Jeffreys the impossibihty of his so-called Bio- 

 donta being that genus, from the absence of the siphonal scar. 

 After a correspondence, I suggested to that gentleman that he had 

 better at once withdraw a supposititious object, and attribute it and 

 the description and figure in the * Annals' to an accidental error that 

 all conchologists may occasionally be subject to. Mr. Jeffreys wrote 

 to me as follows : — " Perhaps you are right about the Biodonta 

 Barleeiy at least to this extent, that I beheve on reconsideration it 

 may be a Biplodonta.'* 



We must now turn our attention to the "Gleanings" in the August 

 * Annals' for 18.58, 3rd series, vol. ii. pp. 117-133. 



Rissoa cimicoides (the R. sculpta of the ' British Mollusca'). This 

 species has not been taken at Exmouth, as Mr. Jeffreys states in a 

 subsequent portion of the " Gleanings :" the error of that habitat 

 has arisen from seeing the specimens received from Mr. Damon on 

 my tablet of Rissoa reticulata (Mont.), as at first view I thought 

 them a mere variety of that species. 



Aporrhdis pes-carhonis. I have always thought it a dwarf variety 

 of A. pespelecani. Mr. Barlee's distinctive characters of these two 

 objects are not of specific value, being dependent as to colour on 

 locality, food, depth of water, and other circumstances. As to the 

 different appearances, in the two objects, of the head, tentacula, 

 proboscis, and foot, they are often very fallacious specific characters ; 

 these organs are in constant vibration, and their changes are incessant : 

 the tentacula often appear to have a line running down their centres ; 

 this, as if by magic, disappears at the will of the animal, until it is 

 reproduced by volition. There are the same capricious phases in the 

 proboscidal apparatus as regards its rotundity and flatness ; but 

 these momentary mutations have no valid specific import ; there 

 must be persistent and substantial differences of structure for the 

 foundation of species. 



Being on the subject of distinction of species, I may mention that 

 some naturalists think that if animals of general resemblance, but 

 which are nevertheless distinguished by certain variations of contour, 

 live together, and preserve their respective differences, from that 

 fact they are probably distinct. I dissent from this supposition in its 

 full extent ; for the animals in question may be typical species, with 

 their varieties, and of course are certainly not distinct. The argument 

 of animals living together neither proves identity nor distinction ; but 

 these gentlemen will not abandon their views of distinction, which is 

 the cause why so many spurious objects, resting on very slight dif- 

 ferences of contour (especially amongst the Chemnitsice, under which 

 term the Odostomice and Eulimellce of some authors are included, 

 as in my ' Brit. Mar. Test. Moll.,' I have shown that the two 



