470 . J- P- J- Ravn. 



straight, almost parallel to the cardinal margin; posterior margin 

 without concavity. A rounded keel from the umbo down to the 

 posterior part of the ventral margin, bounding in front the hind- 

 most, concave region of the vahe. Area low. Surface with con- 

 centric striae. 



Height 9'5 mm, length 19 mm, thickness (of the single valve) 

 5 mm. 



Distribution. North-East Greenland: in a boulder on the 

 south side of Vesterdalen at Danmarks Havn (1 spec). 



Franz Josephs Land: in Middle Callovian. 



Russia: in Lower Volga deposits in Centra) Russia. 



'M\. Macrodon Mylii n. sp. 

 Fl. XXXIV, fig. 1. 



The shell little arched, very oblique, elongated in a diagonal 

 direction. Anterior end narrow, posterior end легу broad; the wings 

 not sharply marked olT from the rest of the shell; the diagonal ridge 

 very much rounded and but little prominent. The beaks but little 

 projecting, almost touching one another. The anterior margin 

 rounded, forming a rounded corner with the cardinal margin. The 

 posterior margin likewise somewhat curved, without concavity, 

 forming a blunt angle with the ventral margin; the size of this angle 

 increasing with age. The \entral margin with quite a slight con- 

 cavity in its Iront part. The surface covered with concentric, regular 

 folds, overlapping like tiles. Here and there — especially on the 

 most strongly arched region of the shell — faint radial furrows. 

 Area little evident, quite unusually low. The hinge not accessible 

 for examination. 



Height 14 mm, length '2'Л mm, thickness (2 connected valves) 7 mm. 



This species is very nearly related to M. Koiiillieri Lab., but is 

 distinct from it by being much less arched and less elongated, hi 

 connection with the former characteristic, the ridge is much less 

 prominent and the area considerably lower. 



I have named the species after the leader of the Danmark Ex- 

 pedition, the author Mylhs-Ehichsex, who met his death on the ice- 

 fields of Greenland. 



Distribution. N о r t h -East G r e e n 1 a n d : lying loose in the middle 

 sandstone region of Store Koldewey Island (1 entire specimen). 



'Л1. Macrodon Hagenii n. sj). 

 PI. XXXIV, figs. 3 and 4. 



One or two, somewhat incomplete shells of a Macrodon greatly 

 resemble M. Keijserlingii , but are so dilferent, that I think it best to 



