250 



mum length. Valves subovate in outline as viewed laterally, somewhat 

 oblique in some specimens but nearly equilateral in others ; higher than 

 long and longest immediately below the middle or near the base: 

 anterior and ventral margins rounded : posterior margin either rounded 

 or obliquely subtruncated and obtusely subangular at its junction with 

 the ventral border below : cardinal border veiy short, and nearly 

 straight on each side of the beaks, but with its outer angles more or 

 less rounded off: umbones angular behind, subcentral, very broad, 

 gibbous and prominent: beaks curved strongly inwards, downwards 

 and a little forwards : posterior area indistinctly defined. 



Entire surface covered with minute and closely arranged radiating 

 ribs. Hinge teeth and muscular impressions unknown. 



Dimensions of an average specimen : maximum length, five milli- 

 metres and a half: greatest height, six mm. 



South side of Maud Island : six badly preserved single valves. 



Judging by its external characters alone, it is not at all unlikely that 

 this shell may prove to be a species of Cardilia (Deshayes), though that 

 genus has not yet been recorded as occurring in rocks of Cretaceous age. 

 In any case the present species is not a true Cardium in the most 

 restricted sense of the word. It may be an extreme variety of the 

 Protocardium Shumardi of Meek and Hayden, which latter will be 

 found described and figured on pages 98-99 of the " Palaeontology of 

 the Upper Missouri." 



PROTOCARDIUM. (Species undeterminable.) 

 Plate 33, fig. 5. 



South side of Maud Island : a small but perfect cast of a species of 

 Protocardium, whose specific relations cannot at present be determined. 

 Its posterior area is marked with rather coarse radiating ribs and the 

 rest of the shell seems to have been concentrically striated. 



NEMODON FISCHERI, d'Orbigny. (Sp.) 

 The synonymy of this shell will be found on page 234. 

 South side of Maud Island : a cast of a left valve. 



LITHODOMUS MAUDENSIS, Whiteaves. 



South side of Maud Island : two single valves of a shell which appear 

 to be precisely identical with the species from the " Lower Shales " of 

 the same Island, described on pages 23? and 238 of the present volume. 



