WHITE.] CROW CREEK FOSSILS. 167 



cral liuiulred feet beneath tlie fossiliferoiis layers at that station, and 

 their equivalents near lUack Buttes Station, some twelve miles distant. 

 It has also been found at the higher horizon just mentioned, west of the 

 Eocky Mountains, and a coui>le of examples were found in the Laramie 

 strata of Crow Creek that seem mnnistakably to belonj;" to this species. 

 Besides this, an example, apparently of this species, has been recognized 

 among some fossils collected by Prof. J. W. Powell in the Canon of 

 Desolation of Green lliver, Utah. It seems to be a comparatively rare 

 species except at the locaUty where it was originally discovered. 



No. 3. Ostrea glabra Meek & Haydeii. 



The same variations exist among the examples of this species from the 

 valley of Crow Creek, where it is quite abundant, that have been noticed 

 on a previous page as prevailing at the locality in the neighborhood of 

 Maynard's ranch ; and the same variations also exist among the examples 

 found here that suggested the reasons before mentioned for regarding 0. 

 Acyomiuf/ensifi'Meek, and i)erhaps 0. suhtrif/onalls Evans & Shumard also, 

 as specilically identical with 0. glabra. The differences are believed to 

 be mainly the result of difference in age, but thej^ were doubtless due, in 

 part, to environment also. 



This species especially characterizes Ko. 4 of the Crow Creek section, 

 but it is also found sparingly in other members, both above and beneath 

 that one. It is also found in strata both above and beneath the one 

 (Xo. 5) that appears to have been a purely fresh- water deposit. 



A curious habit of this species (not altogether unknown in the case of 

 the living 0. virginica Gmelin, and first noticed among fossil oysters 

 bj' Meek, in his description of 0. soleniscits) is that three specimens not 

 unfrequently attached themselves together by the whole length of their 

 under or deeper valves. Instances of these shells being otherwise attached 

 together are not couuiion, and the great majority of them are entirely 

 free, and show little or no indication of haviiig been attached to any i^hing, 

 at least since they were very small. 



The interlamellar layers of prismatic shell structure, commonly observ- 

 able n])on the exterior surface of the shells of living species of edible 

 oysters, have been detected upon all varieties of this fossil species which 

 have been obtained from the Laramie Group. As a rule, however, I have 

 found it more i)lainly shown upon specimens obtained from localities west 

 of the mountains than east of them 5 and in the case of all collections I 

 have foimd it more distinctly shown upon the upper than upon the under 

 valve. The latter peculiarity seems also to prevail in the case of the 

 recent 0. virginica Gmelin ; but in other respects the difference seems 

 to be due to difference in the conditions of their preservation, or rather 

 to the different degrees of their destructive disintegration and not to 

 mere geographical location. It may be mentioned here that I have never 

 been able to detect this prismatic structure upon the shells of any species 

 of the subgenus Alectrgonia., nor upon any of (either Grgpluva or Exoggra. 

 This does not prove, however, that such a structuer does not exist in 

 those members of the Ostreidcv, esi^ecially so since I have hitherto failed 

 to find it ux)ou any species of Ostrea proper which I have found associated 

 with those forms. 



These fossil oysters of the Laramie Group were subject to a i^est that 

 producL'd an effect upon the shells somewhat similar to that which is pro- 

 duced by the burroxAing sponge Cliona upon the shells of living oysters. 

 This pest to the Laramie Ostrea seems not to have been a. burrowing 

 sponge, but more probably a burrowing worm. The burrows are very 

 numerous, of uniform size, not larger than a horse-hair, straight or curved, 



