442 CRETACEOUS PELECYPODA 



probably produced by the posterior branch of tlie pedal muscle, •vrliicli appears 

 to 1)6 confluent with the impression of the adductor ; pallial line entire ; sm-face of 

 valves always radiatcly ribbed and often spinose ; type, Sp. (jcederopus, Linne. 



Eossil species arc known from tlic Lias* upwards, gradually increasing in number 

 and attaining their maximum of development in the present seas. The few species 

 noted from triassic and older deposits are doubtful ; some of them appear to indicate 

 new generic forms, and others have already been transferred to Jlinnites. The 

 attachment of the right valve is in some recent and fossil species very small, and 

 in a few it becomes quite obsolete. When "the young SpondtjU find a suitable 

 place on a solid dead rock they generally become firmly attached, but when they 

 are seated on living coral they become in time quite free, and are found generally 

 placed in an open cavity on the surface of coralline masses. I have myself 

 collected several such specimens on the reefs at Singapore and near the Andaman 

 and Nicobar Islands. 



2(1. As a probable new geiieric form I must mention a peculiar species, Spondylus 

 p7dvinatus, described by Zittel from the Gosau deposits (Denksch. Akad., Wien, 

 XXV, pt. ii, p. 119, pi. xviii, fig. 8). It is a small remarkably thick, transversally 

 ovate shell with a very strong straight hinge line, possessing the external hinge 

 teeth of a Spondijlus, but having the cartilage placed in two pits one beside the 

 other. It is most probable that this species belongs to a genus different from 

 Spondijlus, but as there has been as yet only a single valve found, I shall not 

 venture for the present to propose a new name for it. 



Deshayes has shown that the names Podopsis, Fachytes, and Dianchora have 

 been based merely upon cast, or imperfect, specimens of Spondi/U. In the case of 

 Dianchora, Sow., the beak of the right or attached valve appears to have been 

 broken off, and the opening produced by this break was evidently considered as 

 natiu'ally belonging to the shell. 



3. Fedum, Brug., 1792. Valves generally higher than long, slightly convex, 

 Avith a striated triangular area between the beak and the hinge line, smaller in 

 the left valve than in the right, or larger, valve ; the latter has a deep incision in 

 iVont of the hinge line ; ligament situated in a groove, which passes through the 

 median portion of the area and terminates internally with a projecting cartilage 

 process ; hinge edentulous, muscular scar faint, large, sub-central, roundish, partially 

 confluent with the pedal scar ; pallial line entire, faint ; type, F. spoudijloideum, 

 Gmel., being the only species as yet known from the eastern seas. 



Deshayes censures H. and A. Adams' classification of this genus and places 

 it near Fecten on the ground that the right valve is free. It is not more free than 

 in many Sp)oridyU, and is found resting on corals with the right valve in exactly 

 similar manner as these do, the consequence being that the radiating strijc are 

 generally not developed on the right or larger valve, which also often remains 



* Eud-Deslongchamps has described a few species from these beds, but the hinge-teeth do not as yet appear 

 to have been observed in them. The oolitic species described by the same author, however, bear all the external charac- 

 ters of typical Sponrlyli. 



