74 THE NAUTILUS. 



different structure; it shows a somewhat irregular, more or less 

 oblique and waving, but in general transverse, i. e. circular stria- 

 tion. These undulations are almost everywhere of two different 

 kinds ; larger, averaging in width from O'Ol to 0*022 mill., and 

 smaller, between them, about 0'004, also only 0'007 mill. In many 

 places the two systems are not distinct, and in others the undulations 

 are mingled Avith similar figures of quite irregular form and distribu- 

 tion. On the youngest part, that is the inner end near the bulb, 

 there is nothing of this transverse striation, but in its ])lace a 

 rather thick layer of spindle cells of about 0"006 mill, diameter, 

 more or less transversely arranged, which become the longer and 

 narrower as they are farther from the bulb, and it is evident that 

 the circular striation is derived from these cells. Some acetic acid 

 added brought to appearance the endoplasts (nuclei) in some of the 

 cells, but not very distinctly ; probably the object was not fresh 

 enough. 



It is to be expected that more such specimens will be found, also 

 of other species of Unionidce ; for U. Ugamentinus is hardly the only 

 one to have a byssus long after the embryonic stage. And some 

 points, in which my examination was not sufficient, may then be 

 better ascertained. 



CRITICAL NOTES ON EASTERN TEXAS UNIONIDJE. 



BY H. A. PILSBRY. 



A collection of Uniones lately obtained by the writer from ^Ir. 

 W. L. McDaniel of Tyler, Texas, has given new localities for a 

 number of species, as well as occasion for notes on their synonyms 

 and relationships. 



The species of the southern Mississippi Eiver, described originally 

 from Louisiana, Mississippi and Arkansas, will mostly be found in 

 eastern Texas, and constitute nearly the entire naiad fauna of that 

 region. There are besides these, a few forms common to the whole 

 Mississippi drainage, and found also in the Alabama basin, and a 

 few peculiar to Texas and Mexico, the latter becoming more and 

 more numerous as we travel southward. 



Most of the characteristically Texan species belong to Mississippi 

 River types, a circumstance which first struck me when working up 



