1856.] 277 



It is possible this shell may prove to be only a variety of /. veniricosus, de- 

 scribed by us from the same locality, in a paper read before the Academy last 

 April. It differs, however, from all the specimens we have seen of that species, 

 in being broader, much more compressed and more regularly marked with con- 

 centric undulations. The beaks are also smaller, more elevated above the hinge 

 line, and located a little further back from the anterior margin. It has likewise 

 much the form of /. convexus of Hall and Meek, (vol. 5, Mem. Am. Acad. Arts and 

 Sc. page 386, pi. 2, fig. 2,) but may be at once distinguished from that and /. Sa- 

 gensis of Owen, as well as from all the other species we have seen from the higher 

 formations of Nebraska, by the extreme thinness and entire fibrous structure of 

 the shell. 



Locality and position. Mouth of Judith river, in a sandstone probably the same 

 as No. 1 of the series. 



Inoceramus incurvus. 



Shell ovate, globose or cordiform, equivalve. Beaks located a little behind 

 the anterior end, strongly incurved ; umbonal region very gibbous and much 

 elevated above the hinge line. Surface ornamented with regular distinct con- 

 centric undulations, strongest near the beaks, where they are directed very ob- 

 liquely inward toward the hinge, in passing round from the front to the poste- 

 rior side. Length unknown ; breadth 2 inches ; height 1-80 inch. 



The young of this species, judging from the curve of the undulations near 

 the beaks, must have been of an ovate form, being a little broader behind than 

 in front. As the shell advanced in age it became rapidly more globose, and the 

 beaks curve so strongly inwards, as to bring the umbonal regions of the two 

 valves nearly, if not quite, in contact behind and above the points of the beaks, 

 which are directed away from each other. The substance of the shell is com- 

 posed of an external fibrous, and an internal lamellar portion, the former being 

 generally exfoliated. The surface was probably marked with concentric striae, 

 though not preserved on our specimen. This shell may be distinguished from 

 all the other species we have seen from this region, by its more globose form, 

 as well as by its strongly incurved and gibbous umbones. 



Locality and position. Little Bear's village, between Fort Pierre and Fort 

 Clark, formation No. 4. 



OSTRiEA PATINA. 



Shell circular, oval or ovate, oblique, rather compressed. Superior valve 

 concave above, thin, excepting at the beaks, where it is suddenly thickened and 

 truncate. Inferior valve thicker, concave on the inner or upper side, sometimes 

 auricled ; beaks triangular, flat, slightly curved upwards and turned a little to- 

 wards the anal side. Muscular impression oval or circular, shallow nearest the 

 anal roargin. Surface of both valves marked with imbricating lines of growth, 

 and faint irregular concentric undulations. Greater diameter of largest speci- 

 men (inferior valve) 3-50 inches; smaller do. 3-10 inches; depth of concavity 

 70 inch. 



The usual form of the shell is nearly circular or broad ovate. The buccal 

 side is short and rounded, while the anal border is longer and sometimes (espe- 

 cially in the inferior valve) contracted above just behind the beaks. The areas 

 of both valves are broad triangular, and as usual depressed in the middle, and 

 transversely striate. 



This species is evidently related to 0. vissicularis (Lamk.) of which it may be 

 considered a far western representative. It always differs from that species, 

 however, in having the inferior valve much less concave, more distinctly auri- 

 cled, and never characterized by a posterior lobe. The beak of this valve is 

 also much less curved upwards, being often almost horizontally flattened. We 

 have some fifteen or twenty specimens of this species, in a good state of preserva- 

 tion, all of which are quite uniform in their characters. None of the beaks show 

 marks of the fracture by which the shell became detached. 



Locality and position. Two hundred miles above the mouth of Milk River, No. 

 4 of the series. 



