252 _ MAirUAL OF THE MOLLTJSCA. 



before their "birth, in consequence of wbich the species is not 

 eaten. The tongue of the periwinkle is two inches long ; its 

 foot is diyided by a longitudinal line, and in walking the sides 

 advance alternately. The periwinkle and trochtis are the food 

 of the thrush, in the Hebrides, during winter. The lingual canal 

 of the periwinkle passes from the back of the mouth under the 

 oesophagus for a short distance, then turns up on the right 

 side, and terminates in a coil (like spare rope) resting on the 

 plaited portion of the gullet. It is 2| inches long, and contains 

 about 600 rows of teeth ; the part in use, arming the tongue, 

 comprises about 24 rows.* The dental ribbon oi Ri sella is above 

 2 inches long, and coiled as in Littorina. (Wilton.) 



Fossil, 10 species ? Miocene — . Britain, &c. It is probable 

 that a large proportion of the oolite and cretaceous shells 



Fig. 106. Operculum and teeth of Risella. (Wilton.) TJie central tooth should be 

 pointed, not blunt as in the figui-e. 



referred to turho belong to this genus, and especially to the 

 section tecfaria. 



Sub-genera. Tedaria, Cmier, 1817 (= Pagodella, Sw.), L. 

 pagodus, PI. IX., Fig. 11. 



Shell muricated or granulated ; sometimes with an umbilical 

 fissure ; operculum with a broad, membranous border. West 

 Indies, Zanzibar, Pacific. 



Modulus, Gray. M. Tectum, PL IX., Fig. 13. 



Shell trochiform or naticoid ; porcellanous ; columella per- 

 forated; inner lip worn or toothed; operculum horny, few- 

 whorled. 



* In Fig. 105 is shown the manner in which a gasteropod may be laid out for 

 examination, iinder icater ; the body requires to be fixed, and the cut edges of the 

 mantle to be kept open with needle points. A convenient trough may be made of a 

 plain earthenware soap-dish, by cutting a piece of sheet-cork (such as bootmakers use) 

 to fit the bottom, and fixing it to a piece of sheet-lead of the same size witli a couple 

 of iodia rubber bands. The instruments required for dissecting are simply a pair of 

 fine-pointed scissors, a few broken needles, a penknife, or scalpel, and a pair of forceps 

 vith fine curved points. 



