222 



nearly as oblique as that of the anterior end in some specimens, and 

 almost horizontal in others, and again the truncated margin of the 

 posterior end may be either straight or slightly convex. 



When distorted, as the specimens frequently are, and in almost 

 every conceivalile way, the range of variation in form is, of coui'se, 

 still greater. Some individuals ai-e so much elongated transversely 

 and so narrow in the dii'ection of their height that Pictet's figures 

 of the Thracta Sanctce-Crucis, from the Upper Gault of Switzerland 

 would represent them very accurately, though the normal form, as 

 stated on page 56 of the present volume, is much more like that of 

 the Thracia (or Gorimyd) Nicoleti, of Agassiz, from the Swiss Lower 

 Neocomian. 



This species and Caryatis subtrigona are by for the most abundant of 

 the lamellibranchiate bivalves collected by Dr. G. M. Dawson. 



Pleuromya subcompressa, Meek. 



(Typical form.) 

 Plate 29, fig. 6. 



Myacites {Pleuromya) subcompressa, Meek. — 1873. Annual Report of the United 



States Geological Survey of the 

 Territories for 1872, p. 472. 

 " " " " —1877. United States Geological Sur- 



vey of the 40th parallel, Vol. IV., 

 p. 13(>., pi. 12. figs 6 and 6a. 



?=: Pleuromya jxipyracea, Gabh. — 1869. Palaeontology of California, 



Vol. II. p. 178, pi. 29, ffg. 66. 



Beaks prominent and erect : surface marked by concentric plications 

 " that become nearly obsolete on the po^fterior dorsal region and near 

 the front." 



South side of Alliford Bay : a perfect and well preserved cast of 

 the interior of the right valve. 



The specimen agrees well with Mr. Meek's description and figure of 

 the type of Myacites suhcompressus and with Mr. Gabb's diagnosis and 

 figure of Pleuromya papyracea, or more properly still of Panopea papy- 

 racea, as the broad and deep sinus in Mr. Gabb's typd shows that it is 

 congeneric with the Cretaceous Panopseas. 



