MANUAL OF CONCHOLOGY. 



Monographs of the Acmceidce, Lepetidce, Patellidce and Titiscaniidce. 



Family A CMjEID^E Cpr. 



Acmceidce CPR., Maz. Cat. p. 202, 1856. Tecturidce GRAY and 

 authors. Lottiadce GRAY. Patellidce, in part, of authors. 



Shell patelliform, conical, the apex more or less anterior, the 

 embryonic shell conical, not spiral. Animal having a free branchial 

 plume above the neck on the left side ; radula without median teeth. 



Animals of this family differ mainly from the Patellidce and Lep- 

 etidce in having a cervical branchial plume. 



The shells may generally be known from Patellidce by their dif- 

 ferent texture and the more or less distinct internal border of the 

 aperture. They are never iridescent within. 



They live on rocks and sea weeds, generally at very moderate 

 depths. One species, Acmcea fluviatilis, is known to inhabit brackish 

 water, and a few, like Pectinodonta arcuata, are abyssal. 



The shells are excessively variable, as is usually the casein seden- 

 tary mollusks. 



The author has examined very large suites of specimens, including 

 nearly every species and variety described from the waters of North 

 and South America, both east and west, and of Japan, Polynesia and 

 Europe. The Australian and New Zealand forms are known to me 

 by fewer specimens, and a number of the species of those regions I 

 have not seen. 



In the treatment of species I have aimed to be strictly conserva- 

 tive, reducing no described form to a variety or synonym without 

 the most ample evidence of identity or intergradation of characters ; 

 and on the other hand, I have refrained from burdening science 

 with new names for the vast number of transitional or divergent 

 forms in the collections examined. 



No characters diagnostic of the genera of Acmseidse can be found 

 in the shells. 



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