[NVERTEBKATE PAL^EONTOLOC V. 209 



Mactra Cymbophora!) gracilis. M. & H. 



Plate IT, figs. 18, a, b. 

 Mactra gracilis, Meet and Haydou (May, 1800), Proceed. Acad. Nat. Sci. Philad., XII, 179. 



Shell small, rather thin, sub-ovate, moderately gibbous ; anterior margin 

 rounded, a little shorter and broader than the other; base forming a 

 broad semi-ovate curve, being usually more prominent toward the front than 

 behind; posterior side rather narrowly rounded, or subtrtmcate at the 

 extremity ; beaks moderately prominent, and located slightly in advance of 

 the middle ; escutcheon comparatively large, lance-ovate in form ; surface 

 marked by distinct, regular lines of growth (hinge unknown). 



Length, 0.49 inch ; height, 0.38 inch ; convexity, about 0.24 inch. 



I have placed this little shell provisionally in the genus Mactra, from 

 external characters only, not having seen the hinge or pallial impression. At 

 a first glance, it appears to be similar to our Mactra Warrenana; but, on com- 

 parison, it will be found to be more compressed, less nearly trigonal in form, 

 the beaks being more developed, and not so nearly central, while the anterior 

 outline is more rounded and the posterior side a little more extended; it also 

 differs in being entirely destitute of any traces of a lunule-like area. 



From all the other species of Mactra known in the Upper Mo. rocks, 

 excepting our M. Siouxcnsis, it will be readily distinguished by its more 

 rounded anterior outline, and the absence of any defined lunular space; and 

 from M. Siouxensh, it may be known by its smaller size, much less elevated 

 beaks (which are placed a little farther forward), and by having its dorsal 

 slope, uist in front of the beaks, not near so concave in outline. 



If it should ultimately be decided by conchologists that the name Mactra 

 shall be restricted to some of the other types, and the Trigonella group 

 retained as a distinct genus, it is possible that this and the preceding species 

 may have to be called Trigonella gracilis, T. Warrenana, and T. Siouxensis, 

 as they have the general appearance; and at least two of them, the muscular 

 and pallial impressions of that group, and their hinges are not known to cor- 

 respond to that of Cymbophora. 



Locality and position. — On the Yellowstone River, Idaho Territory, 150 

 miles above the mouth ; in a bed containing a mingling of the fossils of the 

 Fox Hills and Fort Pierre groups, or Nos. 4 and 5 of the Cretaceous. 

 27 n 



