*HiTEAvEs.] LARAMIE AND CRETACEOUS INVERTEBRATA. 37 



iloDIOLA (BraCHTDONTEs) DICHOTOXLi. (X. Sp.) 

 Plate 4, figs. 3 and 3a. 



Shell equivalve, rather sti-ongly convex when adult, thickest on the 

 rounded umbonal ridge, which is moderately prominent and tolerably 

 well defined as far as the middle of the valve>, but which widens and 

 gradually becomes obsolete towards and at the posterior end of the 

 base : maximum thickness about equal to the height in fully grown 

 specimens and about one-fourth le?- than the height in immature one-. 

 Lateral outline somewhat variable in different specimens, the largest 

 being subelliptical with a slightly arcaate base, while tho^e which are 

 not quite full-grown are narrowly subtrapezoidal ; length about twice 

 the maximam height. Anterior side narrow and extremely short, its 

 margin retreating abruptly, obliquely, and more or less convexly 

 ■downwards and inwards ; posterior side considerably elongated and 

 much broader than the anterior, broadest a little behind the middle, 

 its superior border broadly arched in the largest specimens and faintlv 

 and obtusely subangular at the termination of the hinge line behind in 

 smaller individuals, its basal marn-in shallowly arcuate or nearly 

 straight, and its extremity narrowly rounded below the middle. Beaks 

 small, anterior, terminal or very neai-ly terminal. 



.Surface marked by a few concentric lines of growth, and by numer- 

 ous minute, rounded, radiatmg ribs, which curve upwards and outwards 

 and are distinctly dichotomous on the jjosterioi' area, but which are 

 apiparently not sO distinctly dichotomous below the umbonal ridge. 



Dimensions of a perfect cast of the interior of the largest specimen 

 •collected ; length, twenty millimetres ; maximum height, ten mm. ; 

 thickness through the closed valves, seven mm. and a half A smaller 

 Tight valve with the test preserved is eleven mm. in length, and seven 

 in its greatest height. 



St. Hary Eiver, near its confluence with the Belly Eiver, Lt. M. 

 Dawson, 1881, and T. C. "Weston, 188.3; two casts of the interior and a 

 perfect right valve, with most of the test preserved. 



NccuLA CANCELLATA, Meek and Hayden. 



J^ucula cancellata, Meek and Hayden. 1850. Proc. Ac. Nat. Sc. Phil., voL VIII., 

 p. 8.5. 



Meek. 1876. Eep. U.S. Geol. Surv. Terr., vol IX., j.. 102, pi. 28, 

 figs. 13a, b, c, d, e. 



iSt. Mary Eiver, eleven miles above its mouth, G. M. Dawson, 1881 : 



