NO. 1205. SYNOPSIS OF THE XAIADES— SIMPSON. 509 



which the beak sculpture is somewhat concentric and of a peculiar 

 pattern seldom found among living Uuioues, the ridges being nearly 

 straight below and sharply pointed behind in a chevron-shaped pattern. 



I take it that the next step of progress was the development of the 

 genus Pleuyoheina, or forms which are nearly allied to it; though the 

 strange, smooth, elongated, wedge-shaped C nneopsis, with its remarka- 

 bly deep beak cavities and often distorted shells, and the curious 

 Gihhosula seem like Quadrulas making an effort to shape their shells 

 something like those of Unio. In Pleurobema the shell is solid and 

 rather triangular, the beaks are high and typically well forward, the 

 beak sculpture is broken, but most nearly concentric, the beak cavities 

 are shallow, and the young occupy only the outer branchiae. There are 

 great numbers of species I should refer to this or closely allied genera 

 in the oriental tertiaries, and many of them are knobbed. To day the 

 genus is confined to the Mississippi Valley and the northern drainage 

 of the Gulf of Mexico, and all the species are smooth except P. (vsopus 

 and F. rarieosa, which I place in the genus with souie hesitation. 



This genus has an immense development in the Avaters of the Ten- 

 nessee drainage, becoming less abundant as we advance northward, 

 until in the Oliio Eiver and its tributaries only three species are found, 

 Pleurobema elava Lamarck, P. varicosa Lea, and P. (csopus Green. In 

 the Tennessee and Cumberland systems all the species except the two 

 last mentioned belong to the group typified by P. cJava. In the Ala- 

 bama River drainage and a few adjacent streams emptying into the 

 Gulf there is a great development of this genus, but none of the species 

 belong to the clava group. IsTow, if the genus migrated into the Ten- 

 nessee system from the northwest, and it seems quite probable that it 

 did, its presen<?eiu the Alabama Eiver system would seem to prove the 

 theory entertained by some geologists that the Tennessee formerly 

 emptied into the Gulf, flowing down into what is now the Coosa Eiver 

 and into the Alabama. Additional proof of this is fouud in the fact 

 that quite a number of the common Mississippi Valley species, such as 

 U)iio tuherculatus Barnes, U. rectus Lamarck, U. trigonus Lea, TJ. 

 anodontoides Lea, U. ehenus Lea, and others are rnet with, in the 

 Tennessee, the Coosa, and the Alabama Eiver systems, and Unio con- 

 radicus and U. ciomherlandiamcs, Tennessee Eiver forms, occur in the 

 Alabama system. That this southern outflow of the Tennessee ceased 

 a long time ago is shown by the fact that in the Alabama drainage all 

 the northern species of Uniones have a peculiar aspect, and several 

 entire groups of Pleurobema have developed, which are distinct from 

 anything found in the Tennessee. 



It is probable that the trueUnios and their allies were next developed, 

 with simple oval to elongated shells, and moderate beaks showing 

 quite a variety of sculpture. It is generally concentric, being inclined 

 to follow the growth lines, but sometimes the ridges are broken, and 

 again they are somewhat doubly looped. The young occupy the outer 



