CLASS I PELECYPODA 433 



among members of this order show great uniformity in the early stages. 

 But inasmuch as these observations are dependent upon the mode of growth 

 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. — For the purpose of recording compactly the number 

 and character of the teeth in adult Pelecypods, a formula has been suggested 

 by 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 Glycimeris, 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 : — 



Astarte horealis, -rrrTii-n^r- ; Crassatellites antillarum, v, . „,„ ; Venus mercenaria, 

 L -01010 R^iOlOZ??!^ ' K/cOlOm' ' 



R" ioToT (^^^ ^^^^ ^^^® ^ represents the rough area below the ligament). 



In investigating the genesis of the individual hinge teeth in various 

 genera of the Teleodesmacea, 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 h for the posterior tooth Avhen 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, supplemented 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 indicated 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, CA25 = left median cardinal derived 

 from the posterior part of primitive lamella number two ; LA I = ventral 

 anterior lateral, LP III = dorsal posterior lateral, etc. 



Ligament. — The ligament wh-ich unites the two valves, as stated above, is 

 primitively continuous with them as the uncalcified part of the primitive 

 VOL. I 2 F 



