NATURAL SCIENCES OP PHILADELPHIA. 251 



Length, 1*90 inches; height to cardinal margin, 0-90 inch ; to summit of 

 beaks, 1 inch ; convexity, 0-68 inch. 



This is another form that can only be referred provisionally to the genns 

 Chcenomya, since we know nothing of its hinge and muscular and pallial im- 

 pressions. It has the form of the typical species of that group, excepting that 

 its posterior extremity is more compressed, and not so widely gaping. Its 

 most peculiar surface character, is the abrupt deflection of its obscure con- 

 centric ridges, which give it much the appearance of a Goniomya. Indeed if 

 found amongst Cretaceous or Jurassic fossils, we would not hesitate to refer it 

 to that genus. As in some species of Goniomya, the ridges run parallel to the 

 base along the middle of the valves, between the points where they are de- 

 flected. Perhaps we should call it Allorisma hyhrida, but for the present we 

 refer it provisionally to the genus Chcenomya. We know of no species with 

 which it is liable to be confouuded. 



Locality and position. Keokuk division of the Subcarboniferous Series ; 

 Nauvoo, Illinois. 



Genus SEDGWICKIA, McCoy, 1844. 



Sedgwickia (Sangdinolites ?) subakcuata, M. and W. 



Shell elongate, suboval, somewhat arcuate, rather convex in the central 

 auterior, and umbonal regions ; anterior side sloping, with a slightly convex 

 outline from the beaks forward, and rather narrowly rounded at the extremity ; 

 posterior side narrow and compressed above and behind the umbonal ridge, 

 and obliquely truncated at the extremity ; dorsal outline horizontal and con- 

 cave behind the beaks ; ventral margin forming a long gentle convex curve, 

 nearly parallel to the dorsal margin, curving up gradually towards tho front, 

 and very abruptly at the posterior basal extremity. Beaks moderately promi- 

 nent, and placed about one-tkird the entire length of the shell from tho an- 

 terior extremity ; umbonal ridge prominently rounded from the beaks to near 

 the posterior basal extremity. Surface of cast without visible ci>neentrie 

 ridges or other markings. 



Length, 2-20 inches ; height, 0-95 inch ; convexity, 0-72 inch. 



We are by no means sure that this shell belongs to the genus Sedgwickia, 

 as properly restricted to such forms as S. attenuata and S. corruya/a of McCoy, 

 since it is more elongated, and wants the concentric ridges usually seen on 

 these shells. Iu general outline it approaches some species of Cercomya, 

 Agassiz, such for instance as C. striata, from the upper jura, but its posterior 

 umbonal ridge is not so angular, while its surface, judging from internal casts, 

 seems to have been very nearly smooth. As we only know it from casts, 

 nothing can be determined in regard to its hinge, nor have we any means of 

 ascertaining the nature of its muscular and pallial impressions. Possibly we 

 should call it Allorisma subarcuata, though its rather prominent umbonal 

 ridg^, compressed posterior dorsal region, apparently smooth surface, and con- 

 vex anterior slope, without a depression in front of the beaks, give it a kind 

 of Lyonsia like aspect, not seen iu the known species of that genus. 



Locality and position. Upper beds of the Keokuk division of the Subcarbo- 

 niferous series. 



GASTEROPODA. 



Genus HOLOPEA, Hall, 1847. 



Subgenus ISONEMA, M. & W. 



ISONEMA DEPRESSA, M. & W. 



Shell much depressed, considerably wider than high : volutions nearly 

 four, increasing rather rapidly in size, obliquely compressed, with a slightly 

 convex outward slope above, last one subangular around the middle, and 



1865.] 



