WHiTEAVES.] LARAJIIE AXIi CRETACEOUS INVERTEBRATA. 11 



Tvarded by Lit. C. A. "White for comparison, are distinctly ]iyriform in 

 ■outline as viewed from above, the closed valves being very ventricose 

 anteriorly, — the beaks of both valves are gibbous and curved strongly 

 inwards, while the posterior area, though tolerably well defined is small 

 and narrow. In C. perangulata, on the other hand, the outline as 

 viewed from above is ovately lanceolate, both beaks are obliquely flat- 

 tened and the posterior area is large and broad. The external aspect 

 of C. pyrijormis. as !Mr. Meek has pointed out, is like that of a ISWera, 

 ■whereas the outside of the present species has more the look of a Leda. 

 Dr. Dawson states that the beds characterized by a great abund- 

 ance of thi^ species, together with Ostrea glabra, var. Wyomingensis and 

 Corhicula occidentalis (or C. cythcrlfonnis) occur at the very base of the 

 Laramie, and that these deposits may even be regarded as forming a 

 passage between that formation and the summit of the marine Cre- 

 taceous. These beds are most characteristically develo])ed in parts of 

 the south western portion of the district embraced by the geological 

 map before referred to, where they frequently occur in the disturbed 

 strata of the foot-hill region. They have been recognized as far north 

 a^ a few miles west of Blackfoot Crossing on the Bow Eiver. 



Panop.ea si-mulatrix. (X. Sp.) 

 Plate 2, fi^s. 2 and 2a. 



Shell slightly inequivalve, the umbo of the right valve being a little 

 larger and more tumid than that of the left : valves compressed at the 

 sides, thickest on the anterior umbonal slopes and narrowing very 

 gi-adually to the posterior end but more i-apidly to the anterior : pos- 

 terior termination gaping : lateral outline elliptic ovate, the length being 

 fully twice the maximum height inclusive of the beak^^, and the pos- 

 terior side a little longer, narrower and more pointed than the anterior. 

 TJmbones broad, obtuse and depressed' beaks hmall, sidx'entral but 

 placed a little in advance of the middle, that of the right valve curved 

 inwards and downwards with a s^light inclination forwards, that of the 

 left valve curved inwards and a little forwards but ntit downwards : 

 ligament apparently ^hort and external. 



Surface concentrically striated ; inner layer of the te^l not nacreous ; 

 hinge teeth and muscular impressions unknown. 



Length of the most perfect example collected, (the one figured) 

 fifty-two millimetres: greatest height of the bame, twenty-five mm. : 

 thickness of the same, sixteen and a half. 



The specimen from which the aliove description was made and which 



