134 INVERTEBRATE FA UNA OF DAKOTA FORMA TION— WHITE, vol. xvii. 



in form and proportions from tlie one just described to allow its refer- 

 ence to that species. Indeed, it differs so much from the other that it 

 seems to belong- to the type of TJnio alatus, Say. Still, the specimen is 

 too imperfect to allow of a satisfactory specific description, but it is fl<^- 

 ured on plate viii for the purpose of giving as complete a representation 

 as i)0ssible of tlije meager fauna of the Dakota formation, as it is now 

 known. 



Family o r b it l i d ^ . 



CORBULA HICKSII, new species. 

 Plate VIII, figs, fi, 7, 8. 



Shell of medium size, elongate-subtrihedral in mai-ginal outline; pos- 

 terior end prominent and narrow; valves of moderate convexity, not 

 strongly unequal; beaks high and narrow and turning forward; basal 

 margin broadly convex ; posterior margin narrowly rounded ; postero- 

 dorsal margin slightly convex and sloping downward from between 

 the beaks to the narrow posterior end; front margin regularly rounded 

 from the basal margin to the inter-umbonal space ; surface marked by 

 the usual distinct lines of growth; hinge having the typical character- 

 istics of Gorbula. 



Length of the largest example in the collection, which is a left valve, 

 26 mm; height from base to umbo, 16 nun; convexity of the single 

 valve, 6 mm. 



The collection contains an abundance of specimens of this species, 

 all of which are in the condition of natural molds and casts. The fore- 

 going- description has been made from those molds and casts and from 

 artificial casts taken from some of the natural molds. The figures on 

 plate VIII are drawn from artificial casts. 



This form is of the same general type as that of the Laramie Corbula 

 to which Mr. Meek gave the name C. crassitelliformis^ but it is some- 

 what more gibbous and also broader in front. The specific name is 

 given in honor of Prof. L. E. Hicks, its discoverer. 



Class GASTEROPODA. 



Family Ceriphasiid^. 



GONIOBASIS .lEFFERSONENSIS, new species. 



Plate VIII, fig. 9. 



Shell small, slender, sides of the spire approximately straight; volu- 

 tions apparently about 10 in number, gradually increasing in size from 

 the apex to the front; sides of the volutions nearly straight or flat, 

 thus forming the nearly straight sides of the spire; suture linear; sur- 

 face nearly or quite smooth. 



Length of the only specimen discovered, 13 mm; breadth of the last 

 volution, 5 mm. 



