OF CONCnOLOGY. 233 



•cause may, however, be corrected in time and with larger ma- 

 terial. Great care has been taken, repetition has been made 

 frequently in the description of minute characters common to- 

 several species in preference to leaving an opportunity for doubt 

 as to»the characters in each, by silence or cross references. 



A certain geographical agreement in regard to generic char- 

 acters has been observed, which, as far as it offers any grounds 

 for speculation, rather favors the hypothesis of a development 

 of the various forms from a few more simple and more closely 

 allied ancestors. Thus, most of the species from the Straits of 

 Magellan are provided with lateral frills on the foot, and also 

 agree in having the laterals 2 — 1-1 — 2 {Patinella, Nacella) ; 

 those of the British Seas on the other hand have the foot smooth 

 and the teeth 1 — 2 • 2 — 1 [Patella, Patina) ; the Amboynese spe- 

 cies of Acmceidoi agree in possessing two minute uncini instead 

 of one, as in the west coast forms ; the subgenus Collisella at- 

 tains an extraordinary development upon the West American 

 coast, and nowhere else, comparatively few species being known 

 from other seas. On the other hand, certain sporadic forms, 

 like the species of Acmcea occur in widely separated districts. 

 These results, unquestionably due to some law, of which we have 

 as yet only the vaguest intimations, I cannot attribute to the 

 very plausible but highly unsatisfactory doctrine of " natural 

 selection." No doubt the latter has had much to do in the vast 

 field of nature, especially among plants, insects and birds, but 

 there is no reason, except our own natural desire for a theory 

 of some kind, for believing that it has had any appreciable effect 

 on the development of the mollusca. I am impressed with the 

 belief that there will ultimately be found to exist some law or 

 laws far more profound and incisive, and as general and- deep- 

 seated as that elucidated by Mr. Darwin, governing without es- 

 sential exception, the phenomena of evolution all over the globe. 



Anatomical investigations from alcoholic specimens alone are 

 very liable to error, from the rigidity of the parts, and this is 

 especially true in regard to the finer details of the nervous and 

 circulatory system. I have therefore attempted only a general 

 outline of the anatomy of any of the species. The most notice- 

 able differences between the two families Acm?eidge and Patei- 

 lidse are, that in the former a cervical branchia is present, the 

 muzzle is provided with a frill,* there appears (in some species, 

 at least) to be but a single renal organ, the crop is inconspicu- 

 ous and the radula lies in a long double loop on the upper pos- 



* Produced, in the subgenus Acmcea alone, into a lappet or tubercle at 

 each side of the muzzle. 



