432 



MOLLUSCA 



PHYLÜM VI 



Fig. 646. 



Dysodont hinge of 

 Myoeoncha striatula 

 Goldf. LowerOolite; 

 Bayeux, Calvados. 



Vi- 



d'Orb. 



France. 



The Fantodonta are a small group of Paleozoic forms whose dentition 

 partakes of the syiithetic character of the more archaic forms, wliile fore- 



shadowing the f uture 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 AUodesma 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. 

 Ästarte (Fig. 754), Crassatel- 



Dy,odont hinge ot'pachymytilus petasus UteS (Fig. 642) and CorUcula 

 Coral Rag; Ooulange-sur-Yonne, (^jg, 761) are examples. 



The Cydodonta exhibit 

 extreme torsion in their dentition, which cijrves 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 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 

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

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



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

 modonta, 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 Asthenodonta comprise borers and burrowers in which the teeth have 

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

 (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 f eature 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 morpliologie de la coquille chez les lamellibranclies. 

 Bull. Soc. Geol. France [3], 1895-97, vols. xxiii., xxiv. — Vest, W. von, Über die Bildung und 

 Entwicklung des Bivalvenschlosses. Verh. Siebenb. Vereins Naturw., 1895-96, vol. xlviii. — 

 Dali, W. II., On the hinge of the Pelecypods and its development. Amer. Journ. Sei,, 1889 [3], vol. 

 xxxviii. — Reis, 0., Das Ligament der Bivalven. Jahresh. Ver, Vaterl. Naturk. Württ., 1902, vol. Iviii. 



