ALI31ENTA11Y .SYSTEM — OlniXTuriloKAL NOTATION. 
tlie median line at a more acute angle than the lateral teeth usually 
do, aud viewed l»y the light of their development, are the most primi- 
tive form of tooth e.xistent on the odontophore. 
The change in character from the symmetrical median tooth to the 
asymmetrical laterals and marginals is usually decisive and may be 
marked, amongst other things liy the partial or entire loss of the 
endocone or inner ensj) and a corresponding enlargement of the 
mesocone or midille cus]), which with the ectoeone or outer cusp 
may become deejily cleft, this divergence of character becoming more 
and more marked as the outer fringe of the radula is approached, 
aud lieing also more or less closely correlated with the direction or 
curvature of the transverse rows, which is greatly varied in the 
Fic. 
Kici. .c'G. I'll,. 326 . Fiti. rV2'J. 
I )iagrammalic figure:* Nliow'mg ihc varied dire'ctions of llie transverse rows of teeth in dilTereiit 
species. 
Fk;. r>2(». — Tcstacclla haliotidca Urap. I Fig. ,>28 . — ! f elix lapicuia 1,. 
Fit;. ,)27. — I 'cllctia lacustris \ Fig. 521). — Physa/ontinalis 
different si)ecies, an abrupt change in their direction being always 
accompanied by an e(pially marked change in the character of the 
teeth, while if the rows are straight or the change of direction very 
gradual, this change is correspondingly slow, and between the well 
marked laterals and typical marginals several transitional teeth may 
he interposed. 
A Aot.vtion or Formula has been devi.seil to e.xpress, hy means 
of signs ami nundjcrs, the general arrangement and some of the 
more salient characters of the radula of many of our species. 'J'he 
possiliility of doing this is .simplitied by the trail, svorse rows of teeth 
being bilaterally alike, while the longitudinal rows are each .straight 
and contain similar teeth along their whole length. 
The writing of the formula is accomjili.shed by iilaciug a numeral 
to reiire.seiit the central tooth, the particular numeral u.sed according 
with the number of central teeth iire.seiit in a transver.se row; the 
.same cour.se is pursued with the laterals and marginals when these 
are homogeneous in shape and character, the numerals representing 
which, separated hy the sign + , are jilaced at each side of the central 
figure in .similar positions to those actually occuiiied hy the teeth 
