OP MOLLUSCA. 97 



ft Operculum ovate, subspiral, simple. 



2. Rissoa. 



Foot with single dorsal posterior process. Shell ovate, solid ; 

 spire short, smooth or plaited ; aperture ovate, rounded in front ; 

 outer lip more or less dilated, thickened externally. Operculum 

 ovate. 



1. R. auriscalpium, t. 125. f. 13. 



2. R. monodon, t. 125. f. 9 ; Adams, Gen. Moll. t. 35. f. 2. 



3. R.? inconspicua, t. 125. f. 5. 



4. R. abyssicola, Forbes ty Hartley, B. M. t. J.J. f. 3, cop. Alvanea 



a., Adams, Gen. Moll. t. 35. f. 3. 



5. R. Cingillus, Forbes fy Hartley, B.M. t. J.J. f. 4. Cingula c, 



Adams, Gen. Moll. t. 35. f. 8. 



6. R. parva, t. 123. f. 8 ; Forbes #• Hanley, B. M. t. J. J. f. 5. 



7. R. labrosa, Forbes $■ Hanley, B. M. t. J.J. f. 6. 



8. R. rufilabris, Forbes $■ Hanley, B. M. t. J.J. f. 7. 



9. R. Ulvae, Forbes 8r Hanley, B. M. t. J.J. f. 8. Hydrobia U., 



Adams, Gen. Moll. t. 35. f. 10. 



Central tooth of Eissoa membranacea subquadrate, the. base wide 

 and somewhat projecting, with a short lateral process on each side ; 

 cutting edge longly recurved, the tip widely lobed and inciso-dentate 

 on each side ; lateral teeth somewhat transverse, the first produced ; 

 margin before the strong lobe pectinate-dentate ; second and third 

 almost claw-like, the former serrated, the latter serrulated in front 

 (fig- 52). 



Animal of Rissoa striatavrhitish. The rostrum short, rather deeply 

 bifid at the end. Tentacles rather long, subcylindrical, white ; eyes 

 black, very distinct, sessile, a little above the outer side of the base. 

 Foot oblong, slender (interrupted ?), rather tapering behind, sides 

 simple. Opercular mantle simple. Operculum ovate, horny. 



Foot of Rissoa interrupta elongated, rather tapering behind, 

 divided across in front by a transverse interruption, the front division 

 about one-third the length of the rest, separation marked by a pel- 

 lucid line ; the hinder part whitish, with a transparent central longi- 

 tudinal line. The foot very flexible, moving almost independently of 

 the body. Rostrum short, divided into two parts by a central lon- 

 gitudinal slit, and not a radiated plait like Lacuna. Floats like 

 Lymnea. 



Teeth of Rissoa interrupta in forty or fifty rows, 3 • 1 ■ 3 ; central 

 transverse rounded, with three recurved apical dentations, and two 

 on each side of the base. Lateral teeth : inner broad, its inner upper 

 edge dentate, outer linear ; second bifid at the tip, outer simple. 

 Prehensile collar distinct. — Alder. 



