510 GEOLOGICAL SURVEY OF THE TEREITOEIES. ^ 



usually distinct from each other, bat sometimes in contact or a little 

 blended together. 



As the specimens from which the foregoing description was drawn up 

 agree so exactly with the upper valve of Anoniia, both in all internal 

 and external characters, it may be asked, why should there 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 passage 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 saccess. It is true we found a few much 

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

 any i^erforation, or even emargination. These few flat valves found 

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

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

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

 more depressed beak quite marginal. 'No two specimens, however, 

 were found united in such a manner 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 18 inches to 2 feet in thick- 

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

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

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

 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 no sinus or perforation 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 upper 

 part of that system. Again, the flat or depressed specimens, that this 

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

 found, on critical examination, to have their slight obliquity in the same 

 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 probable that they are all the same valve of the shell, va- 

 rying in 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 anomiceformis, 

 of Roemer, (Kreid. Yon Texas, i). 75, pi. ix, tig. 7 a-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 tbtit the obliquity is in the same direction, viewing the two forms both in 

 the same waij either looking at the interior or exterior. 



