70 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 48 



species. Basically, L. hellaplicata novamexicana is distinguished by 

 its more oblique marginal outline, greater convexity of the umbonal 

 region, larger, broader, less numerous plicae, reduced and more 

 centrally situated auricles, much larger posterior auricular sulcus, and 

 more inclined posterodorsal margin. The plicae arise at an earlier 

 developmental stage than in L. hellaplicata hellaplicata. No other 

 Cretaceous species of Lopha are closely comparable. Young shells of 

 these two subspecies are nearly identical (compare pi. 3, figs. 1-9, 

 with pi. 7, figs. 1-3), but average adult shells are readily distinguished. 

 Marginal variants of the subspecies show overlap of many structures. 



Shells of this subspecies are typically found complete, with both 

 valves intact and well preserved. Although they are common at 

 some localities, they have not been observed living gregariously in 

 beds or "oyster reefs," and show no evidence of crowding. The 

 small size of the attachment scar (in some cases too small to be 

 observed, or preserved, on the beak) possibly indicates early detach- 

 ment from the substrate and predominantly a free life. 



Lopha hellaplicata novamexicana is known from three lithofacies: 

 Fine- to medium-grained, well-sorted, cross-bedded marly sandstone ; 

 similar sandstone with an argillaceous cement and abundant carbo- 

 naceous debris; and sandy, argillaceous, limestone concretions or 

 lenses (septarian) in a sandy shale matrix. The first two types are 

 from New Mexico, and the concretions mark its occurrence in 

 Colorado. The only observable morphologic differences between 

 forms from the two States which may be environmentally controlled 

 are (1) the sharper plicae on many Colorado examples (pi. 7, fig. 11), 

 and (2) the tendency for some Colorado specimens to have a ventral 

 "hump," or sharp break in slope about one-third the height from the 

 ventral margin. On the steep slope below this break, the plicae on 

 certain specimens show greater bifurcation than is normal for the sub- 

 species. No such structure was noted on New Mexico specimens. 

 These differences are difficult to evaluate in light of the sparse knowl- 

 edge concerning the paleoenvironment to which this form was sub- 

 jected. Similar irregularities in growth form are produced in some 

 oysters exposed to abrupt changes in rate of sedimentation, in others 

 periodically exposed in intertidal zones. This structure is genetically 

 produced in certain species of Inoceramus, irrespective of environ- 

 ment. It is a gerontic feature of other pelecypods. 



Stratigraphic and geographic distribution. — Lopha hellaplicata nova- 

 mexicana is the oldest known member of the L. luguhris group. It 

 is commonly found in the middle Mancos Shale of New Mexico, in the 



