40 ~The Shell-Collector’s Handbook. 
lobes tree all round, except at the posterior edge, 
where they form two subuliform orifices. 
3. Unio, Philippsson.—Shell firm and _ solid, 
cardinal teeth large, lateral teeth lamelliform. 
4. Anodonta, Lamarck.—Shell thin, hinge eden- 
tulous. 
Famity III.—Dreissenide. 
Shell boat-shaped, somewhat triangular, equivalve, 
furnished with a byssus; umbones placed at the 
extreme end; hinge with small teeth or edentulous ; 
ligament internal. 
5. Dreissena, Van Ben. 
Crass I].—Matacozoa Gastropopa, Cuv. 
Shell univalve or none (internal). Body with a 
distinct head, and two or four tentacles; eyes 
situated at the extremity of the dorsal tentacles, or at 
the base of them; respiration effected by gills or 
lung. 
OrpDER [.—PECTINIBRANCHIATA. 
Shell spiral, external, operculated. Respiratory 
organ consisting of a single pectiniform gill. 
| Famity I.—Neritide. 
Shell semiglobose; spire small, flat, excentric, 
mouth semicircular ; operculum shelly, with a plate- 
like appendage on its under side. 
1. Neritina.—Lam. 
Famity II.—Paludinide. 
Shell cone-shaped, venticose; mouth oval; oper- 
culum concentrically striated. Body oval; eyes sessile © 
' 
