T.ENIOGLOSSATE TEETH. 
■2H.} 
large and broad ; the former may be known as tlie IMajor lateral, to 
mark its superior size, while the latter is distingnished as the Caijitnli- 
form tooth, on account of its fancied resemblanae to the capital of a 
column ; the marginals are very numerous and compact with two 
or more cusps. The formula may be expressed as 
^ + 3 + ^ 
The affinity of the terrestrial air-breathing genera, Helkina and 
Proserpina, to our branchiate Neritina fluviatilis is demonstrated by 
the similarity in character of their dentition. 
The T.ENIOGLOSSA (rati'ia, ribboii ; yAwcro-a, tougue) to which prac- 
tically all our Streptoneurous species belong, are chiefly remarkable for 
the extraordinary length and narrowness of the radula. They are 
usually characterized by bearing seven longitudinal rows of teeth, 
composed of a central tooth, with one or sometimes two somewhat 
Fig. 532. — Transverse row of teeth from the odontophore of r/v/^ara vivipara (L.), highly 
niagnifiedj Northampton, collected by Mr. L. E. Adams, 13. A., and prepared by Mr. J. \V. Neville. 
ample admedian or lateral ones, and one or more, but usually two, 
more slender marginals at each side, all closely serrate or dentate 
at the upper or cutting edge and the outer series often outwardly 
directed. 
This type of dentition is shown by Vivipara vivipara, whose chief 
peculiarities may be expressed by the formula 
To^ + yiro + T + + 10^12 >"90 = 630, 
signifying that there are 630 teeth upon the membrane aiTanged in 
90 transverse rows, each containing a central tooth, flanked on 
either side by two lateral teeth and a single marginal, all strongly 
pectinated upon their cutting edges, the chief feature of which is 
the large (juadrangular central cusp, present upon all the teeth 
except the marginals or uncini. 
A similar general arrangement is found amongst the Nucleo- 
branchs, in which, however, the lateral teeth are more strictly 
aculeate, corresponding to their more carnivorous habits of life. 
