140 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [VoLY. 



of the different classes and families liave been preserved. That is, in 

 such cases all the Conchifers appear to be nearly or quite as well pre- 

 served as any of the other mollusks. In other words, when the circnm- 

 stances were very favorable, all mollnscan shells are well preserved 

 and, when unfavorable, the Ostreidm are found to have been the last of 

 mollnscan shells to yield to their adverse influence. 



A collection of invertebrate fossils that has l)een sent to the Office of 

 the Survey from the Cretaceous strata of Bell County, Texas, unlike the 

 collections from the Western Territories before referred to, shows a very 

 decided deficiency of preservation of the shell-substance of the Gastero- 

 I)ods as well as of all the Conchifers (except the- Ostreidcc) as compared 

 with their associated fossils, although the soft strata from which they 

 came is largely composed of lime-carbonate. In that collection, the Os- 

 treidcc are abundant and varied, embracing nearly all the known generic 

 and subgeneric forms of that family, and all are most perfectly preserved. 

 Their condition in this respect is even better than that of the few asso- 

 ciated arthropomatous brachioijods, although, as already shown, their 

 paleozoic representatives are among the best preserved of all paleozoic 

 fossils. Among those Texan Cretaceous fossils it was observed that the 

 Corals, Echinoderms, Eudistse, and Serpuli©, as well as the Ostreidse, 

 were thoroughly calcified and well preserved, showing a marked con- 

 trast in this respect with the Conchifers and Gasteropods, and in part 

 with the Cephalopods. In the case of the fossils of this coUection, the 

 power of the Ostreidse to resist solution seems to have been shared with 

 several other, but distantly related, fossils ; while the want of such resist- 

 ance on the part of the Conchifers was shared by the Gasteropods and 

 Cei^halopods, instead of being peculiar to the first-named class alone, as 

 it seems to have been in the Hamilton Shales and some other paleozoic 

 strata. 



Since the condition of the marine Tertiary fossils will not be consid- 

 ered in the present paper, it remains only to make a few remarks upon 

 the condition of preservation of the brackish- and fresh-w^ater fossils of 

 the Laramie Group and the succeeding Tertiary groups of the West. 

 The invertebrate faunse of these formations are far less diversified than 

 those of the marine formations, being confined almost entirely to the 

 two mollnscan classes, Gasteropods and Conchifers. These sheUs are 

 found mostly in sandstones and shales which are more or less sandy. 

 They are very often found in the condition of moulds and casts, and 

 sometimes occur as siliceous pseudomorphs ; but they are commonly 

 calcareous and properly fossilized. 'No material difference in the con- 

 dition or perfection of their i^reservation has been observed among the 

 different kinds of these fossils. The environing influences seem to have 

 affected all the families and classes alike, but it was observed that some 

 examples of certain species of Conchifers, which were Ibund in some 

 argillaceous shales of the Laramie Group in Wyoming, had a much thin- 

 ner test than those of the same species which were found in some sandy 



