138 MANUAL OF NATURAL HISTORY. 



6. Family. — Fissure-Limpets (Fissurellidse). Gills 



two, symmetrical, on back of neck ; foot di- 

 lated ; sides with a series of short tentacles ; 

 shell conical ; apex perforate, or margin fis- 

 sured. 



7. Family. — Tooth-Shells (Dentaliidae). Head with- 



out tentacles or eyes ; foot small, conical ; 

 sides simple ; shell elongate, conical ; apex 

 perforate. 



8. Family. — False-Limpets (Tecturidse). Gill single, 



on side of back of neck ; foot with a simple 

 groove on the sides ; tentacles simple ; shell 

 conical, simple. 



9. Family. — Blind-Limpets (Lepetidse). Gills two, 



pinnate, on back of neck ; tentacles subulate ; 

 eyes none or rudimentary ; shell conical, 

 simple. 



10. Family. — Groove-Limpets (Gadiniidse). Gills 



simple, placed obliquely across back of neck ; 

 tentacles folded, expanded ; eyes sessile on 

 middle of their bases ; shell conical, with a 

 groove for vent in front of right side. 



11. Family. — True-Limpets (Patellidse). Gill under 



edge of mantle, forming a more or less com- 

 plete ring round the body; tentacles subu- 

 late ; shell conical, simple. 



12. Family. — Chitons (Chitonidse). Gills in two 



series, one on each side of hind part of body ; 

 head covered by a hood, formed of the united 

 tentacles ; eyes none ; shell formed of eight 

 imbricate pieces along the back. 



