CLASS I PELECY PODA 355 
partakes 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 Corbicula 
(Fig. 712) are examples. ae 
“y: ySO - hinge o 
The Cyclodonta exhibit ,,Pysodont hinge 

2 Aa ir Goldf. Lower Oolite; Dysodont hinge of Pachymytilus petasus, 
extreme torsion wi then Bayeux, Calvados. dOrb. Coral Rag; Coulange-sur - Yonne, 
dentition, which curves out 1). France. 2/3. 
from under the beaks and is 
not set upon a flat hinge plate. Isocardia (Fig. 757), Tridacna, and Cardiwm 
(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 Des- 
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 order Teleodesmacea, and dentally are intimately 
related. Recent studies by Bernard! 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 
! Bernard, F., Sur le developpement et la morphologie de la coquille chez les lamellibranches 
(Bull. Soc. Geol. France [3], XXIII.), 1895, and XXIV., 1896. 
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