252 MANUAL OF THE MOLLUSCA. 
before their birth, in consequence of which the species is not 
eaten. The tongue of the periwinkle is two inches long; its 
foot is divided by a longitudinal line, and in walking the sides 
advance alternately. The periwinkle and trochus 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 
cesophagus 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 25 inches long, and contains 
about 600 rows of teeth; the part in use, arming the tongue, 
comprises about 24 rows.* The dental ribbon of Risella 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 Fisella. (Wilton.) The central tooth should be 
pointed, not blunt as in the figure. 
referred to turbo belong to this genus, and especially to the 
section fectaria. 
Sub-genera. Tecturia, Cuvier, 1817 (= Pagodella, Sw.), L. 
pagodus, Pl. [X., 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, under water ; 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 with a couple 
of india 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 
‘with fine curved points, 
