388 BRUCE WADE 



fifty known from the Ripley formation. This probably does not 

 mean that less than forty species inhabited the Selma stage of the 

 Upper Cretaceous sea in the embayment area, for it is most likely 

 that the widely expanded, quiet, Selma stage of this sea was quite 

 favorable to marine life and that a very large percentage of the 

 Ripley species was inaugurated at that time. It does mean that 

 conditions where so little detritus was being brought in were 

 unfavorable for the preservation of very many Upper Cretaceous 

 species. It may be noted that species of Ostrea, Exogyra, Gry- 

 phaea, Anomia, Paranomia, and Pecten are very common in the 

 Selma at many localities. Also it may be noted that the shells 

 of these species are very hard and resistant, being made up of a 

 dense sort of cryptocrystalline shell material which withstood the 

 corrosive and chemical effects of the sea and the attacks of a minute 

 organism, while they were being buried in the very slowly accumu- 

 lating limy muds. It is true that such species as Ostrea larva 

 Lamarck and Anomia argentia Morton are thin and fragile, yet they 

 are of a compact shell material and more resistant than such 

 thick shells as a number of species of Cucullaea, Cr as satellites, 

 Pugnellus, Volutomorpha, etc. Perhaps this point may be brought 

 out by the study of the shell materials and the occurrence of the 

 well-known species Paranomia scabra (Morton)^, which occurs in 

 both the Ripley and the Selma. This bivalve is made up of two 

 distinct shell materials: a thin, hard, compact, resistant outer 

 layer similar to the shell material of certain species of Gryphaea, 

 Ostrea, Exogyra, etc., and a softer inner layer of prismatic cal- 

 careous shell material which is similar to the shell material of most 

 of the Ripley univalves and bivalves. In the Selma formation 

 only the thin outer layer of P. scabra is found preserved. In the 

 Ripley formation, however, at localities where there are abundant 

 shells of various species unknown in the Selma both the inner and 

 outer layers of Paranomia scabra occur perfectly preserved. Thus 

 if the entire shell of this species were as soft as the inner layer this 

 species too would be unknown in the Selma. 



' S. G. Morton, Synopsis of the Original Remains of the Cretaceous Group of the 

 United States (1834), p. 62. 



