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. For 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 : 



Astarte borealis, p?^^ ; Crassatellites antillarum, c ^;*. l ; Venus mercenaria, 



LxOlOlO RtHllWim rt L el 



P 1Q1 (in 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 b 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, CA1= median cardinal of the right valve, CA26 = 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 which unites the two valves, as stated above, is 

 primitively continuous with them as the uncalcified 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 



