168 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



often brandling', tlie branches being nniform in size "witli the main bur- 

 rows. Although these borrows completely "riddle" those portions of 

 the shell where they are most numerous, they are not quite so destructive 

 in their effects as the burrows of Cliona are. 



A Polyzoan, probably belonging to the genus Memiranifpora, was found 

 encrusting portions of a few shells of this species at Point of Eocks 

 Station, but it has not been observed upon shells from other localities of 

 Laramie strata, except those of Bear Eiver Yalley. This Polyzoan will 

 be noticed under the head of collections from both these localities. While 

 the presence of the Ostrea and Anomia in those strata seems sufficient to 

 prove the saline condition of the water in which they lived, the character 

 of these parasitic or commensal species seems to afford additional i)roof 



No. 4. Volsella (Bracliydontes) regularis White. 



The specimens regarded as types of this species, which is described in 

 the Bulletin of the United States Geological and Geographical Survey 

 of the Territories, vol. iv, p. 707, were discovered in beds Nos. 3 and 5 

 of the Crow Creek section. The species, however, has been recognized 

 at several localities of the Laramie Group west of the Kocky Mountains, 

 as will be noticed on following j)ages. It was the only presumably 

 brackish-water species that was found in bed IsTo. 5, in which all the 

 other discovered species are regarded as of fresh- water habitat. 



No. 5. Anodonta parallela While. 



This species was discovered in No. 5 of the Crow Creek section, and has 

 not since been recognized elsewhere. It has a i)eculiar elongate form, 

 resembling Solemya in that respect, but its generic characteristics are 

 those of Anodonta^ and its associated species are all fresh- water forms, 

 except the Volsella noticed in the preceding»paragraph. It is described 

 in the Bulletin of the United States Geological and Geographical Survey 

 of the Territories, vol. iv, p. 709. With the exception of A. propatoris 

 White, discovered by Prof E. D. Cope in the Judith River beds of the 

 Upper Missouri Eiver, this, so far as I am aware, is the only species of 

 fossil Anodonta that has been discovered in any North American strata. 

 The family, however, to which it belongs is Avell known to be abundantly 

 represented by numerous species of Unio in the western portion of the 

 national domain, from the Jurassic beds to the Bridger Tertiary inclu- 

 sive. The great differentiation of type among even the more ancient of 

 these Uniones, and their similarity to living forms, makes it probable that 

 the genus Anodonta coexisted wdth them all. The i)robable reason why, 

 with the two exceptions named, they have escaped detection, is the well- 

 known fact that their congenial habitat is in still waters and on a soft 

 bottom, and seldom in direct association with any species of Unio. 



No. 6. Unio f 



Only a few fragments of this species were discovered, and these only 

 in No. G of the Crow Creek section. They were too imperfect for specific 

 determination, and are interesting only from the fact that they furnish 

 the only evidence yet discovered of the existence of the genus Unio 

 during the Laramie period east of the Eocky Mountains in what is now 

 Colorado. The x^rob abilities are, however, that they did exist then and 

 there in considerable numbers. 



No. 7. Corhicula Clehurni White. 



Some five years ago Mr. W. Cleburn discovered this species in the 

 valley of Crow Creek, either at or near the same place where I obtained 

 additional specimens in 1877. This species has hitherto been found only 



