INYEltTECUATE PALAEONTOLOGY. JJ)1 



or wide below and acute at the upper end, ascending vertically to the middle 

 of the valves. 



Length, 0.67 inch; height, 69 inch; convexity, 0.50 inch. 

 So much uncertainty exists in regard to the hinge-characters of the 

 genus Thetis that it is not without some doubts that I have concluded to 

 refer the above-described species provisionally to that group. So far, how- 

 ever, as can be determined from Sowerby's rather unsatisfactory description, 

 our shell seems to have a similar hinge; that is, it has no lateral teeth, 

 while there are three well-developed cardinals in each valve, the anterior 

 one of each being trigonoid-conical, and curved upward. The middle tooth 

 in each valve is smallest, and directed obliquely downward and backward 

 across the hinge-plate ; but, contrary to what we usually sec in such teeth, it 

 is most prominent at its upper end. In the right valve, the posterior tooth, 

 judging from the pit for its reception in the left valve, is rather large, quite 

 oblique, and widens from its anterior end downward and backward ; while 

 in the left valve, the posterior tooth, which fits in above that of the other, is 

 still more oblicjue, long, and very narrow. These characters of the posterior 

 teeth do not conform to Sowerby's indefinite character " small." If those 

 of Thetis proper, however, have the same form and direction, their merely 

 small size might not be of generic importance. It will also be observed 

 from the wood- cut at the head of this description, as well as from one of 

 the figures on the plate, that the pallial sinus of our shell, although deep and 

 agreeing well in form and direction with that of Thetis, is rather decidedly 

 shorter ; and that the pallial line in front of the sinus, as well as behind it, 

 presents a curious serrated or crenate outline. 



From the differences above noted, I suspect that although related to 

 Thetis, a critical comparison of our shell with specimens of Sowerby's genus, 

 showing the hinge clearly, would bring to light sufficiently marked differences 

 to warrant their separation. Should this prove to be the case, I would pro- 

 pose for our type the name Thet'wpsis. 



I am not quite sure that our shell is punctate as in Thetis, though some 

 indications of this character appear to be visible under a magnifier on one 

 of our specimens. 



Locality and position. — Near Black Hills, on Cheyenne River ; also near 

 the mouth of Milk River, on the Missouri ; at both localities in the Fort 

 Pierre group of the Upper Missouri Cretaceous series. 



