492 TRANSACTIONS OF THE WAGNER FREE 



The Steiumann Formula. — As we shall have frequent occasions for 

 using the Steinmann system of formulating the teeth, it may be well to ex- 

 plain it. L will represent the left and k the right valve, and the teeth are 

 represented 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 

 laminse which receive the laterals by m, if single; m2, if double. Where two 

 Taxodont rows meet on one hinge-margin, and are not separated by a resilium, 

 a period will mark the junction, as in Area or Malletia. Obsolete or feeble 

 teeth may be represented by the italicized symbol for normal teeth. This 

 system is somewhat extended from that originally proposed by Steinmann, 

 but which I have not been able to consult at first hand. The principles in- 

 volved are essentially the same. Some examples will make it clearer. 



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. 



The formula for the following Teleodont hinges is as follows : Astartc 

 borealis, k7°oioj ;« ^ Crassatella antillarum,^}^^Wl^; 

 Verms mereenaria, j^^to i ol i '" '^'^'^ cdiSt x represents the rough area below the 



ligament ; 

 Cardium isocardia, ^-^^^^^52' Tellina iiUemipta Wood, ktojoii- 



The following illustrate types of the Prionodont order ; i, followed by a 

 number in parenthesis, expresses inconveniently long series in a briefer 

 manner : 



PeetuncuhiS7indatHS,^\\^i:^l/^^''^l; Nucida radiata Hanley, r-i o(io) c?o|l i 



Trigonia margaritaeea, ]^§~ ; 

 Unio batavus, ^pf-^4-^. 



The Anomalodesmacea have few and simple teeth, or are frequently 

 edentulous, so that it is hardly worth while to give more than the single form- 

 ula of Corbiila nucleus ^-^-5. 



Types of Teeth. — The types upon which the shelly processes of the 

 hinge, which we call teeth, are formed, may be classified into several groups. 

 These are not necessarily fundamental ; the teeth, being largely moulded by 

 the dynamics of their situation, change with the influences to which their form 

 is due, and in course of time may become obsolete from disuse {Anodo7i), or 

 modified so as to simulate the teeth of groups with entirely different pedigree 

 [Njtcitla, Miitela ; Plicatula, Trigonia). In general, however, at any given time, 

 the types of teeth are good evidence of the relationship of forms to which they 

 are common, especially if the development from the younger stages of the 

 species compared proceeds along similar lines. In the earlier geological ages 



