356 MOLLUSCA SUB-KINGDOM VI 
in highly specialised Pelecypods, in which the development of teeth is largely 
secondary, care must be taken not to confound these processes with those by 
which hinge teeth were originally initiated in edentulous Protopelecypods. 
Finally, in the Anomalodesmacea we have a tribe of burrowers which have 
preserved to the present day some of the features which characterised the 
edentulous Protopelecypods of ancient geological time. The small teeth of 
the nearly edentulous hinge may sometimes be associated with the submersion 
of the resilium and the development of a chondrophore, but in other cases 
they may be the remnants of hinge teeth acquired in the ordinary way early 
in the geological history of the group. 
Dental Formulae.—¥or the purpose of recording compactly the number 
and character of the teeth in adult Pelecypods, a formula has been suggested 
by Dr. Steinmann, which, somewhat amplified, is as follows :— 
Let L represent the left and R the right valve, and the teeth be repre- 
sented by units; the sockets into which teeth of the opposite valve fit by 
zeros ; the resilium or chondrophore by C; the laterals by 1; the clasping 
laminae which receive the laterals by m, if single; if double, by m2. Where 
two Taxodont rows meet on one hinge margin and are not separated by a 
resilium, as in Pectunculus, let their junction be marked by a period. Obsolete 
or feeble teeth may be represented by the italicised symbol for normal teeth. 
For amorphous, interlocking masses, which cannot be classified as teeth, and 
are of varied origin, the symbol x is adopted. The enumeration begins at the 
posterior end, and the right-hand end of the formula is always anterior. 
Thus, types of Teleodont dentition may be represented as follows :— 
Lm01012 Lmel1017 
, Lar ; Tne 7 5 . 
ee bona SRI Crassatellites antillarum, yuo 
Reiojor (ib this case x represents the rough area below the ligament). 
In investigating the genesis of the individual hinge teeth in various: 
genera of the Teleodesmacea, Messrs. Munier-Chalmas and Bernard have 
adopted the following formula, which expresses at once the origin and 
position on the hinge of the several teeth. In the majority of cases the 
teeth appear to be derived from two primitive pairs of lamellae in each valve, 
one pair anterior and one posterior. Each adult tooth is designated by 
an arabic numeral corresponding to the primitive lamella from which it is 
derived, with a for the anterior and ) for the posterior tooth when a single 
primitive lamella gives rise to two teeth. The laterals are counted from 
below upward in Roman numerals, the odd numbers belonging in every case 
to the right, and the even numbers to the left valve. If it is necessary to 
name a socket it receives the designation of the tooth which occupies it, supple- 
mented by an accent (’). A and P stand for anterior and posterior, L for lateral, 
and CA for cardinal teeth. Finally, if a tooth disappears, its place is indi- 
cated by a zero with an index showing which particular tooth it was. The 
numeration of the cardinals always begins with the right median cardinal 
tooth. Thus, CAl =median cardinal of the right valve, CA2)=left median 
cardinal derived from the posterior part of primitive lamella number two ; 
LA I=ventral anterior lateral, LP I1]=dorsal posterior lateral, ete. 
Ligament.—The ligament which unites the two valves, as stated above, is 
primitively continuous with them as the unealcified part of the primitive 
pellicle secreted by the original shell gland; it is therefore neither external 
nor internal. With its subsequent differentiation, and the thickening of the 
; Venus mercenaria, 
