GEOLOGICAL SUEVEY OF THE TEERITORIES. 493 



these characters, but the two forms can always be readily distinguished 

 when good examples can be had for comparison. 



In its more arcuate form, our shell agrees more nearly with Modiola 

 ornata, Gabb, from the Cretaceous rocks of California ; but that shell 

 diifers very markedly in having its beaks decidedly less, neai-ly terminal, 

 and a more decided and much more prominent lobe in front of them. 

 Another important difference is to be observed in the radiating strise, 

 which on the anterior side of our shell are very minute and closely 

 crowded ; while on that part of Mr. Gabb's species, they are as large and 

 distant from each other as on any .other part of the valves. 



If Scopoli's name Yolsella should be adopted for this genus, as there 

 are some reasons for believing may be the case, this change would 

 require the name of this species to be written Yolsella multilinigera. 



Locality and position. — Cretaceous sandstones, near Coalville, Utah, 

 first ridge, No. 14. 



Trapezitjim micronema, Meek. 



Shell attaining a rather large size, elongate trapeziform, the length 

 being a little more than twice the height, which is about one-third 

 greater than the convexity ; anterior margin very short and round 5 pos- 

 terior margin obliquely truncated above and narrowly rounded below; 

 base nearly straight, on faintly sinuous along the middle, rounding up 

 rather abruptly at each end ; dorsal margin long, straight, and i^arallel 

 to the base ; beaks depressed nearly or quite to the horizon of the dorsal 

 margin, and located one-sixth the entire length of the valves from the 

 anterior margin ; umbonal slopes prominently rounded from the beaks 

 obliquely backward and downward nearly to the posterior basal extrem- 

 ity, while below this convexity a shallow concavity extends from each 

 beak obliquely backward to near the middle of the basal margin. Surface 

 ornamented with numerous very fine, regular, crowded, thread-like radi- 

 ating lines. 



Length, 2,28 inches; height, 1.21 inches; convexity, 0.90 inch. 



I know nothing of the hinge of this shell, and merely place it in the 

 genus Trapezium from external characters. Its form and surface-mark- 

 ings, however, are such as to leave little room for doubts in regard to 

 its relations to that genus or Coralliophaga. 



Note. — Since describing, in Dr. Hayden's Eeportof 1870, (page 301,) 

 an elongated shell, from Utah, under the name Pachyniya (?) truncata, I 

 have been led, by further comparison, to think it far more probably be- 

 longs to the genus Trapezium, and remove it provisionally to that genus 

 under the name T. truncata. In the same way I would remove another 

 shell, described by me in that rei)ort as Tapes Wyomingensis, to Dr. Sto- 

 liczka's genus Baroda, under the name Baroda Wyomingensis. 



Locality and position. — Cretaceous coal-bearing sandstones at Bear 

 Eiver City, on Sulphur Creek, Wyoming. 



CoRBicuLA (Veloritina) inflexa, Mcck. 



Shell longitudinally ovate, a little less than two-thirds as high as long, 

 moderately convex; posterior extremity rather narrowly rounded, or 

 apparently sometimes faintly subtruncated ; anterior very short, subtrun- 

 cated, or more or less sinuous in outline, just in advance of the beaks, 

 on the abrupt forward slo^je above, and ratlier abruptly rounded below ; 

 basal margin semi-ovate, or semi-elliptic; dorsal margins inflected and 

 forming a long convex slope from the umbonal region posteriorly ; beaks 



