Descriptions of Species. 31 



iiinbone ; i^osterior uiiilional margin elongate, sloping posteriorly 

 at an angle of al:)out oO degrees from the unibone ; this margin 

 is straight when viewed laterally, but bends strongly outward 

 toward the gaping posterior margin, with which it unites l)y a 

 gentle curve ; surface marked l)y strongly rounded ribs and 

 grooves, subconcentric, sinuous and uniting with each other at 

 a common groove which is parallel with the anterior margin. 



Only a single poorly preserved cast of a right valve of this 

 s[)ecies has l)een found, the anterior portion of which is not 

 visible. It was collected in the heavy conglomerate which 

 marks the Ijase of the Comanche Series at its contact with the 

 Carboniferous formation, on S3-camore creek, Burnet county, 

 Texas, near the crossing of the Burnet and Travis Peak roads. 

 The species is named for Dr. Otto Lerch, of the Louisiana State 

 Geological Corps, who, as a Texan, has made valunl)le contri- 

 butions to the knowledge of his State. 



Pleuiomya (?) henselli sp. nov. 



Plate IV, Figs. 1, 2. 



Shell elongate-elliptical ; length, three and one-quarter inches ; 

 height, two inches; umliones anteriorly- subcentral, round, and 

 not i)rominent; anterior margin strongly rounded, continuing 

 1)y curvature into the pallial margin ; pallial margin elongate, 

 slightly curved ; posterior margin sul)-truncatc, slightly gaping ; 

 anterior uml)onal margin sinuous, slightly gaping ; posterior 

 um1)onal margin about one-third longer than anterior ; interior 

 and liingc mechanism not seen. 



The generic position of this species is very doul)tful; l)ut, in- 

 asmuch as it is one of the most characteristic casts of tlie (ilen 

 Rose beds in the Colorado River section, it is important that it 

 should be here figured. The species has a rcsem1)lance to Thra- 

 cia mi/;vforini>< White, but differs in some details. 



Isocardia (?) medialis (Conrad). 



Plate II, Figs. 4, 5; plate III, Fig. 0. 



Cdrdiiim. viediale Conrad, U. S. and Mexican Boundary Rejiort, 

 vol. I, p. 149, plate iv, figs. 4rt, b. 



Conrad descrilicd this form as follows: 



"Cordate equilateral, ventricose; base profoundly and nearly 

 regularly rounded ; beaks prominent; [)osterior margin truncated, 

 direct." 



