62 Zoological Society : — 



respect the fossil differs from the recent sturgeon in having a shorter 

 and deeper trunk, in the greater vertical exf)anse and wider diver- 

 gence of the lobes of the caudal fin, in the median position of the 

 dorsal fin, and in the absence of dermal plates on the back, belly, 

 and flanks. Before describing the cranial anatomy, the author 

 points out certain homologies between the head-plates of the recent 

 sturgeon and the epicranial bones of the teleostean fishes, more 

 especially with reference to the parietals, mastoids and frontals ; and 

 explains that these conclusions have resulted from the examination 

 of the inner table of the skull, where the relative position and propor- 

 tions of the component plates are constant, however much the outer 

 or dermal layer may vary. 



The remainder of the memoir is devoted to detailed descriptions 

 of such parts as are preserved in the several specimens ; and the 

 author concludes by stating as the result of his investigations, that 

 Professor Agassiz was right in referring the liassic fish to the 

 Sturionidae ; that in some respects it evidenced a transitional form 

 between the latter family and the m.ore typical ganoids ; that its 

 food was similar to that of the existing members of the family, but 

 that it was procured in a tranquil sea, rather than in the tumultuous 

 waters frequented by sturgeons of the present time. 



ZOOLOGICAL SOCIETY. 



February 9, 1858.— Dr. Gray, F.R.S., V.P., in the Chair. 



On a New Genus of Mytilid^e, and on some Distorted 

 Forms which occur among Bivalve Shells. By Dr. J. 

 E.Gray, F.R.S., V.P.Z.S. 



"We have for several years had some specimens of large Myti- 

 lidcB in the Museum Collection which I have always regarded as the 

 types of a distinct genus, but have deferred from time to time their 

 publication, as I was informed that Dr. Dunker and others were en- 

 gaged on a monograph of the family. Dr. Dunker having described 

 the species without forming it into a group, I have therefore brought 

 it before the Society, and at the same time make some observations 

 on a peculiarity which the species presents. 



Stavelia, n. g. 



Shell inequiv.alve, inequilateral, subtrigonal ; umbo anterior ; the 

 front of the ventral edge sinuous, the flatter valve with a broad ex- 

 panded lol)C on the front of the ventral margin, the more convex one 

 with a deep sinuosity to fit the lobe of the other valve. Anterior 

 adductor scar distinct, oblong ; posterior roundish ; submarginal 

 scar parallel to the edge of the shell, entire. Hinge toothless. Li- 

 gament and cartilage linear, marginal, rather short. 



Periostraca laminate, with elongated flat linear or tapering pro- 

 cesses. 



This genus differs from Mytilus in the inequality of the valve and 



