Mr. W. King on a new genus of Palceozoic Shells. 315 



fossils will remain uncertain : from their geological age and their 

 resemblance to a magnesian limestone species which certainly 

 does not possess any appearance of teeth, they may for the present 

 be grouped with the latter. Whether any of these edentulous 

 shells characterise the lias and the oolite is a question on which 

 it is difficult to decide: the valve which Goldfuss has figured 

 under the name of Lutraria decurtata (Petrefacten, tab. 153. fig. 

 3 c), appears to be unprovided with a tooth, but it must be observed, 

 that in the corresponding valve of Panopcea gibbosa and some 

 other species this part is scarcely developed ; it is the opposite 

 one which possesses the largest tooth. 



If the genus Panopcea be allowed to comprise species with co- 

 nical (P. norwegica) or expanded teeth (P. gentilis, P. gibbosa), 

 whether gaping little or much posteriorly, their being dentigerous 

 will form a good distinguishing character by which to separate 

 them from the edentulous Panopsea-like shells of the carboni- 

 ferous rocks : add to this, the disparity between their cartilage 

 fulcra, which in the former are constant in form and direction, 

 while in the latter they are extremely variable in these respects. 

 On a consideration of these differences it is proposed to group 

 the carboniferous shells under the following genus : — 



Allorisma*, nobis. 



Syn. — ? Myacites, Schlotheim ; Hiatella {sulcata), Fleming ; Sangui- 

 nolaria {gibbosa in the Min. Conch.), J. de C. Sowerby and others ; 

 Unio {Urii in Prestwich's Memoir), J. de C. Sow. ; Lutraria 

 {prisca), Goldfuss ; Pholadomya {elongata in Silliman's Journal, 

 vol. xxix.), Dr. S. G. Morton; ? Mya {rotundata in Murchison's 



• Silurian System '), J. de C. Sow. ; ? Posidonomya {transversa), 

 Portlock. 



Order Lamellibranchiata, Blainville. 



Suborder Dimyaria, Rang. 



Family PHOLADOMYiD^f, nobis. 



Gen. Char. — Both valves furnished with a cartilage fulcrum 

 elongated in the direction of the cardinal line, and which varies 

 breadthwise in different species, from being horizontal with the 



* From oLKKoloSi variable, and 'i^sia/^oi, support — expressive of the variable 

 nature of the cartilage support or fulcrum. 



f Blainville 's family .Pylorida? is so heterogeneous that I have taken 

 the liberty to propose that of Pholadomyidce for the genera Pholadomya, 

 Allorisma, Panopcca, Lysianassa (Munster), and Cercomya (Agassiz). In 

 doing this, I am aware of the anatomical differences between Panop&a and 

 Pholadomya, so ably worked out by Valenciennes and Owen ; but they do 

 not appear to be of such importance as to warrant the placing of these two 

 genera in distinct families. 



Y2 



