CLASS I 



PELECYPODA 



431 



the nomenclature he proposed has required revision. Several groups or 

 kinds of teeth can easily be distinguished. These are not necessarily funda- 

 mental, since 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 {Anodon), or modified so as to 

 simulate the teeth of groups with widely different pedigree (Nucula, Mutela, 

 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 

 under comparison proceeds along similar lines. 



The modifications of the hinge now generally recognised are as follows : — 



In the Taxodonta the hinge is composed of alternating teeth and sockets, 



mostly similar, and frequently forming a long series, as in Area (Fig. 643, A) 



A 



Fig. 643. 



Taxodont hinges. A, Area, with external ligament. B, Leda, 

 with internal resilium. 



Fir;. 044. 



Schizodont liinge. 

 Trigonia pectinata 



Lam. Recent ; Aus- 

 tralia. 



or Leda (Fig. 643, B). The Schizodonta have heavy, amorphous, variable 

 teeth, often obscurely divided into sub-umbonal (pseudocardinal) and lateral 

 (posterior) elements, as in Trigonia (Fig. 644), Unio (Fig. 645), and Schizodus. 



In the Isodonta the original taxodont 

 provinculum is often replaced in the adult 

 by a hinge structure derived from two 

 ridges (the "auricular crura") originally 

 diverging below the beaks. This becomes, 

 in the most specialised forms, an elaborate 

 interlocking arrangement of two concentric ^' 

 pairs of teeth and sockets, which cannot 

 be separated without fracture, as in Spon- 

 dylus (Fig. 718). In less sj^ecialised forms, 

 such as Pccten, the provinculum becomes 

 obsolete, and the crura only partially de- fS^^^^ 

 velop. 



The Dysodonta of Neumayr was originally a heterogeneous group, and the 

 term is now restricted to that division having a feeble hinge structure, whose 

 origin is more or less palpably derived from external sculpture impinging 

 upon the hinge line, as in Myoconcha (Fig. 646), Pachymytilus (Fig. 647), and 

 Crenella. 



The preceding groups, together with the edentulous Solemyacea, constitute 

 the order Prionodesmacea, which is knit together by community of descent 

 still traceable in their anatomy. 



Fig. G45. 



Schizodont hinge of Unio stnchei NenmajT, 

 showing pseudocardinal and lateral teeth. 



