ON A NEW EXOGYRA FROM THE DEL RIO CLAY 



AND SOME OBSERVATIONS ON THE EVOLUTION 



OF EXOGYRA IN THE TEXAS CRETACEOUS 1 



BY EMIL BOSE 



Nearly twenty years ago attempts were made to subdivide 

 the Texas Cretaceous, basing stratigraphical zones on the 

 occurrence of oysters, especially Gryphaeas and Exogyras. 

 That such a subdivision must be doomed to failure will be 

 evident to any paleontologist on account of the extreme 

 variability of oysters, Exogyra included. But these at- 

 tempts are interesting, in so far as they showed that Ex- 

 ogyras occur in most of the Cretaceous beds of Texas and 

 that all these may be related to each other. Exogyras occur 

 practically from the lowest stratum of the Trinity form- 

 ation, which corresponds to the Aptian, to the highest Cre- 

 taceous, the Navarro beds; which in their upper part, the 

 higher Escondido beds, certainly represent the Maestrich- 

 tian. 



In order to show the evolution of Exogyra in the Texas 

 Cretaceous a thorough collection of specimens of this genus 

 in the different beds and a redescription of the known 

 species would be absolutely necessary; first, because many 

 forms are still undescribed; and second, because in most 

 of the described species the development from the smallest 

 to the mature individuals has not been sufficiently taken 

 into account. The very small specimens are often the most 

 important ones, because their ornamentation is frequently 

 quite different from that of the full grown individuals and 

 demonstrates from which forms in lower beds the species 

 has branched off. Often the ornamentation on the very 

 first part of the beak of mature specimens shows also this 

 original ornamentation, but generally the beak has been 

 corroded by the movement of the shell in the water or by 

 the grinding effect of sand in currents. In these cases the 

 beak will appear smooth, although the younger specimens 

 show a decided ornamentation. In Exogyra the evolution 



x The manuscript for this paper was submitted in June, 1919; 

 the bulletin was issued in September, 1919. (Ed.) 



