122 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106, 



collected in the Cretaceous sandstone on East Canyon creek, Utah, 

 where it is associated with Cardium pauperculum and other character- 

 istic species of the Colorado fauna. Some much larger casts from about 

 the same horizon, the first ridge in the Coalville section, are believed 

 to belong to the same species, because of their close agreement in form, 

 when allowance is made for the slight distortion of the type. One of 

 these larger specimens is figured. 



In the Pugnellus sandstone of Huerfano park, Colorado, one of the 

 more abundant species is a small Mactra that in size and form is very 

 like the above-mentioned Coalville specimens, and I think it best to 

 describe and figure it under the same name, although one can never be 

 certain that a well preserved shell, showing the surface characters, be- 

 longs to the same species with an imperfect internal cast from a distant 

 locality. 



The description of the form as above given serves very well for the Col- 

 orado examples, excepting that the most of them are proportionally more 

 convex and many are not quite so elongate anteriorly. There is a large 

 distinct lunule, lanceolate in form, bordered by an impressed line that 

 extends from the beak to the extreme anterior end. Behind the angu- 

 lation of the posterior umbonal slope there are one or two other faint 

 radiating lines, the posterior of which forms the boundary of the escutch- 

 eon. Both the lunule and the escutcheon are marked by regular, dis- 

 tinct concentric lines. The remainder of the surface bears only lines of 

 growth that are very faint and closely arranged, excepting toward the 

 ventral border of adult specimens, where they become nearly equal in 

 size to those on the lunule. 



The hinge structure, so far as seen, is that of a Mactra; there is a car- 

 tilage pit, in front of which is an a -shaped tooth, and the lateral teeth 

 are well developed. The pallial sinus is short and rounded. 



This species has some resemblance to small examples of Mactra war- 

 renana, which occurs in the upper portion of the Montana formation, 

 but M. emmonsi is a more depressed form with a smoother surface and 

 slight differences in outline, which are regarded of specific importance, 

 especially when considered in connection with the difference in size and 

 in stratigraphic position. 



Mactra htjerfanensis n. sp. 



PI. xxvn, Figs. 14 and 15. 



Shell thin, depressed, subelliptical, nearly equilateral; anterior bor- 

 der regularly and rather narrowly rounded ; posterior border forming 

 a slightly convex regular slope from the beak to the subangular poste- 

 rior end; base forming a broad regular curve; beaks small, projecting 

 slightly beyond the cardinal border. Surface smooth, marked only by 

 faint lines of growth. 



This species varies considerably in length and in convexity. The 

 specimens selected for figuring, representing the extremes of variation, 



