8 ACM^A. 



Species of the genus Acmsea are found in the littoral and lamin- 

 arian zones of nearly all seas, except the waters adjacent to the con- 

 tinent of Africa. 



The shells are subject to even greater mutations than the Patel- 

 lidse, and species are correspondingly difficult to define and limit. 

 More than any other shells, these must be studied with constant 

 reference to not only habitat geographically, but station as well. 

 For an exact knowledge of the group we must therefore wait until 

 observations on the species are made with especial reference to their 

 modes of life and surroundings. Such data should be attached to 

 every limpet collected. 



Acmcea has been divided by Dr. W. H. Dall into a number of 

 groups which may be tabulated as follows : 



A. Muzzle with lappets ; no uncini ; formula of teeth (3.0.3) 0, 



Acmcea s.s. 



B. Muzzle without lappets, uncini present, Collisella Dall. 



a. formula of teeth 1 (3.0.3) 1 Collisella s.s. 



h. formula of teeth 2 (3.0.3) 2 Collisellina Dall. 



The type of Acmcea is A. mitra Esch., dentition pi. 42, fig. 82 ; of 

 Collisella, A. pelta Esch., dentition pi. 42, fig. 81 ; and the type 

 of Collisellina is A. saccharina L., dentition pi. 42, fig. 83. 



It is practically impossible at present to group the species of the 

 entire world according to anatomical characters, or to decide to what 

 extent these divisions will prove applicable to the entire series. 



The most convenient and in most cases the most natural division 

 of the genus is geographic. Thus considered, the species fall into 

 six groups : 



I. North Atlantic and Arctic. 



II. Western coast of North America. 



III. Western coast of South America. 



IV. West Indies. 



V. Japan. 



VI. Indo-Pacific. 



(VII. Species of unknown habitat.) 



Of these groups, the second has great aflfinity to the first and fifth ; 

 the fourth may be regarded as derived from the second during the 

 early tertiary period. 



I. Species of European Seas and the North Atlantic. 



Many specimens of all of the species of this region, have been 

 examined by me. 



