454 CRETAeEOUS PALEONTOLOGY. 



the 'beak may be convex. Surface of both valves marked with 

 more or less squamose concentric lines oi growth. 



The dimensions of an average lower or convex valve are: 

 width, 6g mm. ; length, 66 mm. ; convexity, 38 mm. 



Remarks. — ^This species approaches most closely to G. convexa, 

 but it is almost always a smaller shell and it may be distinguished 

 by several more or less constant characters. Perhaps the most 

 important of these characters is the smaller and deeply concave 

 upper valve; in those specimens preserving the two valves, the 

 upper one is sometimes scarcely more than one-half the length of 

 the lower. It is possible that in the living animal this shell had 

 a thin calcareous extension tO' the margin of the lower valve, but 

 in the fossil specimens this has often been entirely destroyed, if it 

 ever were present. This species also differs from G. convexa in 

 the much less conspicuous posterior auriculation of the shell ; this 

 is more or less a variable character in both species, and sometimes 

 a strongly auriculate individual of G. dissimilaris does not differ 

 materially from one of the less strongly auriculate individuals of 

 G. convexa, but in such a case the two specimens would be suffi- 

 ciently distinct by reason of the greater concavity of the upper 

 valve of G. dissimilaris. The average conditions, however, of the 

 two forms are widely separated. The shells of the two species 

 differ materially in thickness, that of G. convexa usually grows to 

 be ponderously thick in old individuals, while that of G. dissimi- 

 laris never attains more than a moderate thickness, and is often 

 exceedingly thin and fragile for this group of molluscs ; in any one 

 locality all the shells of the species are of much the same character 

 as regards thickness, and it is among the individuals from those 

 localities where the shells are thinner that the upper valves are 

 smallest ; it is possible that this difference in the shell within the 

 species itself is due to differences in the amount of calcium car- 

 bonate in the waters available for shell secretion by the molluscs, 

 or to difference in depth of the waters in which it lived. Besides 

 these differences in the characters of the shells, the two forms 

 occur at entirely different horizons, and in no case have they been 

 found associated together. Whitfield apparently referred this 



