178 



THE AMERICAN NATURALIST [Vol. XL 



Genekai. Description 



Acinwa tediulinaUs , like the limpets in fjeneral, has a somewhat 

 dish-shaped shell, (Figs. 1 and 2), rouo-hly conical in section and 

 with a mouth (corresponding to the base of the cone) which is 

 broadly oval thongh with the anterior part a trifle narrower than 

 the posterior, llie apex of the shell lies not over its center but 

 about one third of the distance from the anterior end. It may be 



worthy of mention that the true, or doc-oglossate Hnij)ets and the 

 keyhole limpets (Fissurelli(he) dillVr in thi^ ropcct fn.ni all the 

 other widely dissimilar genera to w liidi fonn (it ^licll is connnon 

 and in which the apex, though varying in j^osition in ditlVrent 

 forms, is never anterior. The condition in Acma>a is a secondary 

 one for Boutan ('98, y). 1809) finds that in A. virgrnea the apex is 

 at first posterior and only in course of development assumes the 

 adult position. 



In color the shell is usually yellowish gray marked with ra<liat- 

 ing stripes or tesselations of dark brown. The extent and the tint 

 of the markings vary greatly and they are sometimes almost or 

 quite absent. The liyposlraciini. or inner layer of the shell, stops 

 a little short of it- c.lur tliar on the internal a-peet the marking- 

 appear as a nari-ow border, 'i'liis I nii(ler.>taiid lo be the "more or 

 less distinct internal border of the aptM-ttu'e." mentioned by Tryon 

 and Pilsbry ('91, p. 5) as a character by which the Acmieidne are 



