GASTEROPODA. 



129 



operculum is half egg-shaped, and of annular formation ; 

 the nucleus lying near the inner straight edge, and having 

 an inner central process. Animal, with two blunt, cylin- 

 drical tentacula, and the angle of the region of the mouth 

 drawn out into feeler-like processes, these two being 

 scarcely shorter than the others; the head not muzzle- 

 shaped in front ; the eyes lie remotely behind the ten- 

 tacula over the back, visible through the shell. 



Rissoa glabra of Alders, from the English coast, belongs 

 to this genus ; the animal and the operculum are so pecu- 

 liar that they will probably form a family alone. 



Family 24.— VERMETACEA. 



The foot of these animals is often club-shaped, and not 

 adapted to creeping; the head has the form of a short 

 snout, and frequently bears four feelers; the eyes are 

 placed on the outer base ; the arming of the tongue, &c, 

 is unknown. The shell is attached, tubular, irregular; 

 the apex only regularly spiral. — Marine. 



Vermetus. Adanson. (Vermicularia Schum. ; Serpu- 

 lorbis Sassi; Serpula Lin., part) — Shell thin, tubular, 

 only slightly twisted; aperture round; lips 

 united; the interior often divided by par- 

 titions; operculum circular, concave exter- 

 nally ; shell attached to foreign substances. 

 Animal, head round in front ; four tentacula, 

 two above with the eyes externally at the 

 base, and two below between the head and vermetusjumbri- 



* calls. 



foot, which are strongly contractile ; the foot 

 projects above the head, is wart-shaped, and truncated at 

 the end; one gill lies on the left side, and a second 

 is rudimentary. — 1 species ; also fossil. 



K 



