32 Hill Paleontologij of the Trinity Division. 



To this I would add : 



Variable in shape; shell thick, concentrically striatc ; beaks 

 inturned as in Isocardw ; anterior muscular impression very 

 prominent, angular on anterior side ; posterior muscular impres- 

 sion faint, large in area, almost indistinguishable; posterior 

 margin truncate in normal specimens. Surface marked by 

 strong concentric rugose lines. 



The generic position of this form is not satisfactory, because 

 the hinge structure has not been found. It is clearly not a 

 Cyprina, although sometimes similar forms are referred to that 

 genus by paleontologists in Europe ; neither is it a Ctirdiam as 

 described by Conrad. 



This is one of the most numerous, conspicuous, and character- 

 istic species of the Glen Rose beds ; it occurs asshelless casts by 

 the thousands throughout the vertical and geographic extent in 

 Texas and Arkansas, beginning in the lowest fossiliferous hori- 

 zon and extending to the top. 



A few specimens were found about ten miles west of Glen 

 Rose, upon which fragments of the thick calcified shell structure 

 were preserved, showing it to have a concentrically striated sur- 

 face and the anterior cardinal margin and beaks to be as in Iso- 

 cardia, as shown on plate vii, fig. 1. 



The form varies greatly in shape, owing to the compression 

 and distortion it has undergone in the strata. One of the ex- 

 treme variations from the normal is illustrated on plate ii, fig. 5. 



Natica (?) texana Conrad. 



U. S. and Mexican Boundary Survey, p. 157, plate xiii, figs. 

 la, b. 



This species is of rare occurrence in the beds at Glen Rose. 

 It is not a Natica,, but inasmuch as only casts are preserved its 

 generic position cannot otherwise be stated. 



Viviparus (Natica?) cossatotensis Hill. 



Arkansas Geological Survey, Annual Report 1888, vol. ir, p. 

 130, plate iii, figs. 4, 4a, 5, 5a. 



This form was originally described in my Arkansas Report 

 from an isolated locality in Scvier county. Many consider it a 

 Natica, but it is too imperfect to assert its generic position with 

 certainty. The writer inclines to the belief that it is n fresh or 

 brackish water form. 



