530 MOLLUSCA 



DIVISION 1. DOCOGLOSSA. 



Limpets. Shell conical and not spiral; 2 osphradia; 1 auricle; 1 

 ctenidium or none; mantle margin often fringed (Patella)', jaw median 

 and unpaired; no operculum; radula very long, with but few hooked 

 teeth in each transverse row: 3 families, with 1,500 species, all marine 

 and occurring mostly in shallow water. 



FAMILY ACM^IDAE. 



Ctenidium present, projecting from the left to the right; apex of 

 shell usually inclined anteriorly: 1 species on the Atlantic and several 

 on the Pacific coast. 



ACMJEA Eschscholtz. Shell solid and not iridescent within; ten- 

 tacles rather long and cylindrical; anus on the right; lips frilled and 

 elongated at the lower anterior corners; radula with 2 

 centrals; mantle margin not fringed: about 40 species. 



A. testudinalis (L.). Common limpet (Fig. 822). Shell 

 depressed, smooth, gray or greenish in color, with dark 

 brown stripes radiating from the apex; inner surface 

 whitish, with a large brown spot; length 35 mm.; width 

 25 mm. ; height 9 mm. : circumpolar, south to Long Island 

 Sound and to Sitka ; common north of Cape Cod on rocks 

 i n shallow water; a smaller, narrow, spotted variety 

 (A. alvea Conrad) is attached to eel grass; Europe. 

 A. patina Eschs. Shell 53 mm. long, 46 mm. wide, and 18 mm. 

 high; surface with radiating striations, grayish; inside white, with a 

 large, brown central area: entire Pacific coast; the commonest Cali- 

 fornian limpet. 



DIVISION 2. RHIPIDOGLOSSA. 



Shell usually spiral, the spire being in most cases low and little coiled ; 

 jaw paired; radula long; teeth, 1 central, 3 to 9 laterals, and very many 

 crowded marginals; 1 or 2 ctenidia; 2 auricles; 2 kidneys: 14 families, 

 in the sea, fresh water, and on the land, grouped in 2 subdivisions. 



Key to the subdivisions of Ehipidoglossa: 



Ci Shell and mantle with slits or holes ; 2 ctenidia 1. ZYGOBRANCHIATA 



a, Shell without slits or holes ; 1 ctenidium 2. AZYGOBKANCHIATA 



SUBDIVISION 1. ZYGOBEANCHIATA. 



Shell and mantle with a marginal or an apical slit, or with marginal 

 holes; 2 ctenidia (Fig. 821, A) : 3 families, all marine. 

 Key to the families of Zygobranchiata described : 



a Shell patelliform (like Fig. 822) 1. FISSURELLIDAE 



a. Shell spiral and flat ; aperture very wide 2. HALIOTIDAB 



