432 



MOLLUSCA 



PHYLUM VI 



nor the latter 

 one gi'oup. 



Dysoflont hinge of 

 Mi/ocoiich(.i striiUula 

 Goldf. LowerOolite; 

 Bayeux, Calvados. 



Vi. 



d'Orb. Coral Rag; Coulange - sur ■ 



France. '■^/■. 



■.i- 



The Pantodonta are a small group of Paleozoic forms whose dentition 

 partakes of the synthetic character of the more archaic forms, while fore- 

 shadowing the future teleo- 

 dont 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, 

 three in any 



Fir:. 646, 



pjj. g^^ Astarte (Fig. 754), Crassatel- 



Bysodont hipge of Paehymytilu!< petaM.s UteS (Fig. 642) and Corbicula 



i-Yonne, (Fig. 76 1 ) are examples. 



The Cydodonta exhibit 

 extreme torsion in their dentition, which curves out from under the beaks 

 and is not set upon a flat hinge plate. Isocardia (Fig. 806), Tridacna and 

 Cardium (Fig. 801) 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 sjjecialised 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 

 Macfra, making the hinge more complicated or efficient. Cytherea (Fig. 809), 

 Madra (Fig. 824), Venus mercenaria and Tivela are examples. 



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

 modonfa, which he founded on such types as Madra 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 Astlienodonta comprise borers and burrowers in which the teeth have 

 become obsolete from disuse. Corhula (Fig. 828), Mya (Fig. 827) and Pholas 

 (Fig. 833) 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 featvu'e is explained by the dynamics of Pholad existence. 



The above groups form the orderTeleodesmacea, and dentally are intimately 

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



^ Bernard, F., Sur le developpement et la morphologie de la coqiiille chez les lamellibrancbes. 

 Bull. Soc. Geol, France [3], 1895-97, vols, xxiii., xxiv. — Vest, W. von, tlber die Bilduiis uiid 



Entwicklnng des 

 Dull, W. 11., On t 

 xxxviii. — Rcis, O. 



Bivalveiisclilo.sses. Verh. Sieheiib. Vereins Naturw., 1895-96, vol. xlviii.- — 



he hinge of tlie Pelecypods and its development. Ainer. Journ. Sci., 1S89 [?>], vol. 



Das Ligament der Bivalven. Jahresh. Ver. Vaterl. Naturk. Wiirtt., 1902, vol. Iviii. 



