ALIMENTARY SYSTEM — ODONTOPHORAL NUTATION. 
•268 
themselves ; thus the formula 20 4- 4 + 1 + 4 + 20, would indicate that 
ill each transverse roiv there was one central tooth Hanked by four 
lateral and twenty marginal teeth, each series of a practically 
homogeneous character. Sometimes the marginals are so minute and 
comiiact or so numerically variable that it is difticnlt or impossible to 
state their exact number ; in such cases the sign of infinity ( oo) may 
appropriately be used to indicate them. When, however, the teeth 
composing a series are markedly heterogeneous in form and general 
character, the numerals representing them may he used individually 
or in suitable combinations, so that their dissimilarity may be 
emphasized, thus cc + 1.1.1.1. + 1 + 1. 1,1.1. + x would signify that 
there was one central tooth, with four dis.similar laterals, and 
an indefinite number of homogeneous marginals at each side. 
The number of trainsverse rows should also be added to the formula, 
preceded by the sign x , which will often enable the total number of 
teeth on the radula to be readily ascertained if desired, as although 
their number is approximately constant in full-grown individuals of 
the same species, it varies very considerably in different groups and 
even in different species, and is not at all dependent on the size of 
the animal, for Ihjallnia luctda has only about 945 denticles, whereas 
Helii' ohvoluta, a species of about the same dimensions, has 15,300. 
Increased precision in notation may be attained by the use of 
larger or smaller figures to indicate the relative sizes of the teeth and 
also by placing beneath the numeral rei)resenting the number of teeth 
in each group, a second series to denote the number of reflections or 
cusps upon the individual teeth ; if these consist of a variable number 
of minute and numerous pectinations, they may he indicated by the 
sign of infinity oo , as already suggested for the representation of 
marginal teeth when numerously present; where, however, the number 
of teeth or pectinations do not exceed 20, and the amount of variation 
is not accurately known, it will be more satisfactory to give the ascer- 
tained number as near as may be with the marks of increase or decrease 
as most appropriate, thus 14 < would convey the information that 14 
or more teeth or pectinations were present ; 1 4 that the variation 
was towards a lesser number, while cfd^ would indicate that the 
(piantity present ranged above and behjw the number indicated, thus : 
+ § + -«■ + §+ l-i- X 38 = 950, 
the full dental formula for Hyfdlnia alUarki, shows that species, in 
the particular preparation examined, to possess 950 teeth, contained 
