ALIMENTARY SYSTEM — ODONTOPHORAL TEETH. 
261 
The Radular Teeth or denticles, according to their position and 
character, may also he classified as Central, Admedian or Lateral, and 
iMarginal, and are all reflected posteriorly, the reflected points or 
cusps being known as Mesocones, Ectocones or Endocones, according 
to the relative position they occupy on the individual teeth. 
The Central teeth, which occu^iy the rachis or longitudinal mid-line 
of the radula, are symmetrically formed, but do not originate by the 
coalescence of two adjoining laterals, except perhaps among the 
Limnanda', in which the central teeth may, as in Planorbis, show no 
central cu.sp, but two eijually developed lateral cusps. The convex 
anterior margin of each tooth often overlaps the base of the pre- 
ceding tooth, and is usually trifidly reflected posteriorly, the median 
reflection or cusp being known as the Mesocone and the smaller side 
cusps as the Ectocones. 
Fig. 521.— Central or Median Tooth of Helix aspcrsa JMiill., highly magnified. 
b. basal plate ; r. reflected portion ; in. mesocone or middle cusp, with c. cusp or cutting point ; 
cc. ectocone or outer cusp with cutting point. 
Figs. 522, 523. — Admedian or lateral teeth of Helix aspcrsa Miill., from right and left sides of 
and adjacent to the median row. 
b. basal plate ; r. reflected portion ; in. mesocone or middle cusp, with c. its cusp or cutting 
point; cn, endocone or inner cusp ; cc. ectocone or outer cusp. 
Fig. 524. — Intermediate or Transition tooth of Helix asj>crsa Miill. 
in. mesocone, showing endoconic bifurcation ; cc. ectocone or outer cusp. 
Fig. 525. — Marginal tooth or Uncinus of Helix aspersa !Mull. 
bifid mesocone ; ec. bifid ectocone. 
The Admedian or Lateral teeth occupy the Pleura; or longitudinal 
areas, adjoining the central one, and are usually asymmetrical modi- 
fications of the median teeth, becoming more and more primitive, or 
unlike the symmetrical central teeth the further they recede from 
them. Tlie anterior margin is strongly reflected posteriorly, the term 
Mesocone being retained for the reflection, representing the middle 
projection of the rachidian series ; the term Ectocone designating the 
cusp nearest the outer margin, while the cirsp on the inner side of 
the tooth, nearest the centre of the radula, is called the Endocone. 
The Marginal teeth or LLicini occupy the longitudinal outer 
areas of the radular membrane, and in many species show a wider 
base and more numerous denticulations ; they also tend to approach 
