CLASS I 



PELECYPohA 



356 



Bayeux, 



Vi- 



Calvados. 



Dysodont hinge of /'<// </n// /'/.- /*/ 

 d'Orb. Coral Rag ; Coulange - sur - Yonne, 

 France. 2/ 3 . 



Isocardia (Fig. 757), Tridacna, and Canlinin 



]>;u t.ikes of the synthetic character of the more archaic forms, while fore- 

 shadowing the future Teleodont types. In this group the laterals may 

 exceed a pair in a single 

 group, which is never the 

 case in the modern types. 

 Orthodontiscus and Allodesma 

 are examples. 



The Diogenodonta are the 

 modern and perfected forms 

 in which there are differen- 

 tiated lateral and true car- 

 dinal teeth upon a hinge 

 plate, the former never ex- 

 ceeding two nor the latter 

 three in any one group. 

 Astarte (Fig. 705), Crassatel- 

 lites (Fig. 593), and CorUcula 

 (Fig. 712) are examples. 



The Cydodonta exhibit 

 extreme torsion in their Goidf. Lower poute; 

 dentition, which curves out 

 from under the beaks and is 

 not set upon a flat hinge plate. 

 (Fig. 752) are examples. 



In the Teleodonta are found the most highly perfected types of hinge. 

 The characters of the less specialised forms hardly differ from those of the 

 Diogenodonta, but they are placed here on account of their obvious affinities 

 as shown by other characters. The most specialised forms add to the ordinary 

 cardinal series of the Teleodesmacea (10101) either a roughened area, as in 

 Venus ; a series of extra cardinals, as in Tivela; or accessory lamellae, as in 

 Mactra, making the hinge more complicated or efficient. Cytherea (Fig. 760), 

 Mactra (Fig. 775), Venus (mercenaria), and Tivela are examples. 



Several of these forms were included by Neumayr in a group called Ues- 

 modonta, which he founded on such types as Mactra under a misapprehension 

 as to the character of the hinge ; almost all of the others were included in 

 his Heterodonta, which, construed strictly, would take in all dentiferous 

 Pelecypods, since the alternation forming its essential character is inseparable 

 from the possession of functional teeth. 



The Asthenodonta comprise borers and burrowers in which the teeth have 

 become obsolete from disuse. Corbula (Fig. 779), Mya (Fig. 778), and Pholas 

 (Fig. 784) are illustrative types. In the last-named a remarkable develop- 

 ment of the sub-umbonal attachment of the mantle has produced a myophore 

 which is sometimes wrongly interpreted as a tooth. The exceptional develop- 

 ment of this feature is explained by the dynamics of Pholad existence. 



The above 'groups form the c-rder Teleodesmacea, and dentally are intimately 

 related. Recent studies by Bernard l as to the genesis of individual teeth 

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

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



1 Bernard, F., Sur le developperuent et la morphologic de la coquille chez les larnellibraiiches 

 (Bull. Soc. Geol. France [3], XXIII.), 1895, and XXIV., 1896. 



