314 Mr. W. King on a new genus of Palaeozoic Shells. 



that each valve is provided with a cartilage fulcrum which varies 

 considerably in different species. In some it resembles that of 

 Pholadomya and Panopcea, being more or less folded upon itself, 

 and somewhat horizontal with the dorsal plane of the shell, and 

 having the appearance of a callosity ; in others its free margin 

 sinks a little below the hinge line ; while in the remainder it be- 

 comes unfolded and lamelliform, and is directed so much down- 

 wards as to hang vertically in the dorsal cavity of the shell. The 

 cartilage consequently passes from an external to an internal 

 position. 



It will be seen from this description that the cartilage fulcra 

 gradually change from one extreme to the other, — a fact which, 

 viewed in connexion with the uniformity of their external cha- 

 racters, completely prevents these shells being grouped into more 

 genera than one. 



In rocks of the Devonian and the Permian system similar fos- 

 sils have been discovered ; and the Triassic deposits of continental 

 Europe, especially the muschelkalk, yield somewhat analogous 

 forms (Myacites). Through the Permian and Triassic shells a 

 passage is opened into the genera Panopcsa (Lutraria gibbosa, Sow.), 

 Lysianassa (Mya v-scripta, Sow.), and Cercomya (Sanguinolaria 

 undulata, Sow.*) of the Jurassic system. Associated with the 

 latter occurs the genus Pholadomya, which though it agrees with 

 the shells just mentioned in its cartilage fulcra, differs, perhaps 

 with the exception of Cercomya, in being unprovided with teeth : 

 in this respect the genus Pholadomya corresponds with the so- 

 called Sanguinolarias of the carboniferous rocks, but in general 

 it may be readily distinguished from them by the presence of 

 ribs proceeding from the umbones. 



Many of the Jurassic Panopseas differ from those at present 

 existing, not only in their being slightly gaping, but in their teeth 

 being expanded and in being concave on the upper side : in some 

 species (Panopcea [Lutraria) gibbosa, Sow.) the teeth appear to be 

 but rudimentary, or mere folds of the cardinal margins beneath the 

 umbones ; in this respect they offer an approximation to the car- 

 boniferous fossils. It will thus be obvious how difficult it is to 

 draw the line of demarcation between the genus about to be pro- 

 posed and the Jurassic Panopaeas, and how perplexing it is to de- 

 cide as to the genus in which certain intermediate forms ought to 

 be placed. This last remark applies to the fossils termed Mya- 

 cites, which until it is known whether or not they are furnished 

 with teeth, their identity with Panopcea or with the carboniferous 



* Mr. Morris, in placing this shell in Anatina (vide Cat. Brit. Fossils, 

 p. 77), has overlooked the fact of this genus having the cartilage placed on 

 internal spoon-shaped fulcra. Sowerby's figure of the fossil (Min. Con. 

 tab. 548. fig. 1.) shows that the fulcra are external. 



