TAYLOR : ON THE VARIATION OF MOLLUSCA. 29 1 



parts of the moUusk taken without reference to the shell, offers an 

 extremely slight and variable criterion of specific difference," but 

 it is evident from the context, that he had only the external or outer 

 appearance in mind when enunciating that statement, and did 

 not intend it to apply, to the anatomy. He did not profess to 

 be conversant with the organization of the mollusca, and gives 

 no information on the subject in his great work. 



A forcible illustration of the necessity and value of internal 

 examination, is shown by the establishment — mainly by the 

 labours of Mr. Ashford — on solid, reliable and durable grounds of 

 Testacella sciitulum Sowerby, as a sound and perfectly distinct 

 species. This form has heretofore been considered as only a 

 slight var. of T. haliotidea, from reliance having been previously 

 placed exclusively on the external and visible characters of the 

 animal and shell. 



On the other hand, it will probably lead us to unite together 

 the Helix hispida and H. conci/tna, which Dr. Jeffreys and other 

 able conchologists regarded as distinct and separate species, as no 

 structural differences between them have been brought to light 

 by the anatomical examinations that have as yet been made. 



Anatomy will thus I trust lead us to sound conclusions in 

 arriving at the limits and relationship of species and varieties, 

 for though by Darwinian philosophy, species are considered to 

 be unstable and changeable, they may for our purpose be con- 

 sidered as permanent and fixed. 



Mr. Marrat, however, who has made a life-study of Nassa, 

 in his article on the variations of the shells belonging to that 

 genus, expresses some opinions on species and varieties, and he 

 makes the statement that " the great genus Nassa is one shell in 

 an endless variety of forms," but I am unable to accept as a 

 proof of the soundness of his statement, the facts he brings 

 forward as in a sense establishing his position. The only points 

 that appear to me to be proved are the extreme variability of 

 Nassa, and the necessity of applying definite names to what we 

 may still call species and varieties, for although Mr. Marrat ex- 



