354 MOLLUSCA SUB-KINGDOM VI 
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 Arca (Fig. 594, A) 

Fic. 595. 
Fic. 59 Schizodont hinge. 
1G. 994. Trigonia pectinata, 
Taxodont hinges. A, Arco, with external ligament. B, Leda, Lam. Recent; Aus- 
with internal resilium. tralia. 
or Leda (Fig. 594, B). The Schizodonta have heavy, amorphous, variable 
teeth, often obscurely divided into sub-ambonal (pseudocardinal) and lateral 
(posterior) elements, as in T’rigonia (Fig. 595), Unio (Fig. 596), and Schizodus. 
In the Jsodonta 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- 
Fie. 596. dylus (Fig. 669). In less specialised forms, 
Schizodont hinge of Unio Stachei, Neumayr, Such as Pecten, the provinculum becomes 
showing psendocardinal and lateral | teeth.” obsolete, and | the crura /only)) parmally 
develop. 
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 Jyoconcha (Fig. 597), Pachymytilus (Fig. 598), and 
Crenella. F 
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. 
The Pantodonta are a small group of Palaeozoic forms whose dentition 

