276 [November, 



joined together at the hinge, when the valves are united, while the opposite 

 angles terminate immediately under the beaks. Muscular impressions shallow, 

 ovate or oblong, and having a very slightly elevated line passing from the lower 

 inner border of each up towards the beaks. Lateral teeth of the hinge oblique, 

 but not horizontal. The beaks are curved at right angles to the hinge, and 

 slightly angular behind. A neat little shell, not apt to be confounded with any 

 other species known to us. 



Locality and position. Quite abundant at the mouth of Milk River, in formation 

 No. 4 of the series. 



Mytelus subarcuatus. 



Shell narrow ovate, arcuate ; posterior end compressed and rounded ; dorsal 

 edge regularly arched and sharp ; base nearly straight or slightly arcuate, 

 scarcely carlnated ; the two edges converging towards the nearly straight, 

 pointed beaks, at an angle of about 35. Surface unknown. Length 1-03 inch: 

 breadth -27 inch ; height '36 inch. 



Tlie beaks are nearly or quite terminal, and appear to have been straight or 

 but slightly declining. Our specimen being only an internal cast, shows none 

 of the surface characters, excepting traces of concentric lines of growth. The 

 dorsal border looks as though it had formed a regular curve, without any angle 

 marking the posterior termination of the hinge. 



This species is very near M. lanceolatus of Sowerby, (Min. Con. pi. 439, fig. 2,) 

 but is proportionally wider behind, and not quite so pointed at the beaks. It 

 is about intermediate in form between that species and M. edentulus of the same 

 author. 



Locality and position. Dog River, near mouth of Judith River, from sandstone 

 supposed to be No. 1 of the series. 



Gervilia subtortuosa. 



Shell thick, lanceolate, tortuous and laterally curved. Beaks terminal? 

 pointed ? posterior end narrow, the widest part being at the back end of the 

 hinge ; hinge line straight, forming an angle of about 20 with the longitudinal 

 axis of the shell; ligament fossaj about six, nearly equalling the spaces between. 

 Surface unknown. 



Our specimens of this shell consist of fragments, from which it is impossible 

 to make out a complete diagnosis of the species. None of them show the hinge 

 teeth, and they have all lost the surface markings. The largest fragment we 

 have seen (which is imperfect at both extremities) is four inches in length by 

 1-65 inch in height at the posterior end of the hinge. It retains 2*'70 inches 

 in length of the hinge, which is "44 inch in thickness, though some of the outer 

 lamina; are wanting, and shows five of the ligament fossae, of which there may 

 have been one or two more. 



When viewed on the side, the outline of this shell is much like G. avicidoides 

 of Sowerby, (Min. Con. vol. 6, page 16, pi. 511,) but it is much more curved 

 and twisted, though not so much so, nor so obtuse at the anterior extremity as 

 Gervilia (^Gastrochcena) tortuosa of the same author. So far as we know, this is 

 the first species of the genus described from the cretaceous rocks of this country. 

 Prof. Tuomy informed us he has found a species in Alabama, but as he has not 

 yet published a description of it, we have no means of instituting a comparison. 



Locality and position. Three hundred miles above Fort Union, on the Missouri, 

 No. 4 of the series. 



Inoceramus pertenuis. 



Shell very thin, broad ovate or oval, somewhat compressed ; cardinal border 

 straight ; anterior end rounded ; posterior extremity broad, obtusely rounded ; 

 base broadly curved. Beaks near the anterior end, rising above the hinge line, 

 nearly touching. Surface irregularly and rather faintly undulated, marked 

 with fine lines of growth. Length about 4-30 inches; height 3-40 inches; 

 breadth 2-20 inches, 



