552 T^HE PALEONTOLOGY OF MlNiJ^SO^A. ^^,,„,,^i, .^^undata. 



Formation and ZocaZ^.-Upper part of the Trenton limestone, Minneapolis, Minnesota. The 

 illustrated specimen was found by Mr. A. D. Meeds and Icindly given hy him to the author. The other 

 a much smaller and doubtful cast, belongs to the survey collection ^nd bears the museum register 

 humber 8329. 



Vanuxemia rotundata Hall. 



PLATE XXXVIII, FIGS. 8— W. 



Gypricardites rotundata Hall, 1861. Bept. Supt. Geol. Sur., Wis. p. 29; 1862, Geol. Kept. Wis., 



vol. i, p. 38, fig. 7, and p. 437. 



CypHeardites rotundatus (part.) Whitfield, 1874. Geol. Rept; Wis., vol. iv, p. 208. (Not the speci- 

 men illustrated -pi. V, flg. 11-which belongs to V. suberectaVLmcz.) 



This species is very similar to V. dixonensis Meek and Worthen, and in another 

 direction quite as much like V. subereda Ulrich. Still, as it is very constant in its. 

 peculiarities, and not at all difiBcult to distinguish, it should be recognized as a dis- 

 tinct species. From the first it differs in being shorter from the beaks to the base 

 and therefore circular rather than ovate in outline. The form of the casts, the only 

 condition in which the species has been observed, is more erect, the beaks curving 

 much less forward so that the auterior margin projects considerably beyond them. 

 The anterior sulcus is on the whole stronger, the pallial line more distinct, and the 

 average size of the shells little more than half what it is in V. dixonensis. In other 

 respebts, including the hinge, the two species are practically indentical. Hall says 

 there are two posterior lateral teeth in each valve, and Whitfield one or two, but in 

 all the specimens seen by. me (about flftyX their number was not less than two and 

 oftener three. 



Compared with V. suberecta, a form that was united by Whitfield with V. rotun- 

 data, the latter is found to be more oblique, with the anterior end longer and more 

 rounded al}ove, the sulcus stronger, more curved, narrower, and without the small 

 ridge which is included in the sulcus in that species. Nor is the anterior boundary 

 of the sulcus, especially beneath the muscular scar, so much thickened. There are 

 furthermore some differences in the backs of the two species, the hinge line being 

 less sunken, the dorsal ridges more obtuse, and the outline, in a side view, straighter 

 and even a little concave behind the beaks in some casts of V. suberecta. The hinge 

 of the latter is not fully known, but so far as our knowledge goes, it adds another 

 difference in the greater obliquity of the cardinal teeth. The survey collection con- 

 tains two examples (Mus. Reg. No. 8321) of an unusually convex small variety of this 

 species. Four views are given of one of these on plate xxxviii. 



Formation and locality. — Very common in the "Lower Blue Beds" of the Trenton formation at 

 Janesville and Beloit, Wisconsin. A few specimens from the upper part of the limestone at Minneapolis, 

 are doubtfully referred here. 



Mus. Reg. Nos. ?5101, 8319, 8321. 



