510 GEOLOGICAL SURVEY OF THE TERRITORIES. 



iisnolly distinct from each otlier, but sometimes in contact or a little 

 blended to^etber. 



As the specimens from which tlie foregoing description was drawn up 

 agree so exactly with the upper valve of Anomia, both in nil internal 

 and external characters, it may be asked, why should tLiere be any 

 doubt in regard to the shell really belonging to that genus! The rea- 

 son for doubt on this point is, that among thousands of perfect separate 

 valves, not a single one with the usual perforation for the passiige of 

 abyssal plug, such as we always observe in the under valve of Anomia, 

 is to be seen. While at the locality, diligent search was made, among 

 vast numbers of well-preserved separate valves, for some traces of such 

 a perforated valve, without success. It is true we found a few much 

 depressed valves, some indeed quite flat, but all alike entirely without 

 any perforation, or even emargination. These few flat valves found 

 usually have the cardinal margin a little more truncated than the others, 

 but agree well with tliem in size, general outline, texture, surface-mark- 

 ings, and their muscular impressions, as well as in never having their 

 more depressed beak quite marginal. Iso two specimens, however, 

 were found united in such a^nanner as to show that the flat and convex 

 valves belong to the same individuals. Yet it is worthy of note that if 

 these flat and slightly convex valves are not the opposite valves of the 

 ventricose deeper ones, we must view all of the countless thousands of 

 individuals, almost entirely composing a bed IS inches to 2 feet in thick- 

 ness, as upper valves only, (assuming the fossil to be an Anomicij) and 

 believe that, by some unaccountable agency, these were deposited here 

 together, without the admixture of any of the opposite valves. At oue 

 time I was inclined to think this shell might belong to Morris and Ly- 

 cett's genus Placunopsis, which, it will be remembered, agrees with 

 Anomia., excepting in having nosinusori^erforation in the lower valve ; 

 and it is possible that this may be the case. There are, however, some 

 objections to this view. In the first place, Placunopsis has not, I be- 

 lieve, been found above the Oolite, while the bed in which the fossil 

 under consideration occurs, if even Cretaceous, must belong to the upi)er 

 part of that system. Again, the flat or dej^ressed specimens, that this 

 view would require to correspond to the under valve of Placunopsis, are 

 found, on critical examination, to have tlieir slight obliquity in i\\Qsame 

 direction as that of the deeper valves, instead of the reverse, as would 

 be the case if the two forms are the opposite valves of the same shell.* 



From these facts, and the exact agreement in the number and ar- 

 rangement of the muscular impressions in all of these specimens, it 

 seems more i^robable tbat they are all the same valve of the shell, va- 

 rying iu their convexity in different individuals. This, it is true, would 

 leave unexplained the cause of the entire absence of any of the oppo- 

 site or lower valves, whether it be an Anomia or a Placunopsis. Still, 

 it is well known that currents sometimes perform curious freaks, in the 

 way of assorting from other material and depositing objects of like 

 kinds together. 



In most respects, this shell agrees so nearly with Ostrea anomicvformis, 

 of Roetner, (Kreid. Von Texas, p. 75, pi. ix, tig. 7 «-e,) from the Upper 

 Cretaceous of Texas, that I have even suspected that it might be the 

 same. His figures, however, represent a rather more attenuated beak 

 of the convex valves, and less obliquity, both in the convex and flat 

 valves. A much more important difference, supposing his figures to be 



* I mean tbiit tlie obliquity is iu the same direction, viewing the two forms both in 

 the same ivay either looking at the interior or exterior. 



