32 ///// — Palfontolocpi of the Trhiifij Divisfon. 



To this I would add : 



Varial)]e in pliape ; shell thick, concentrically striate; healcs 

 inturncd as in Isocardia ; anterior muscular impression very 

 prominent, angular on anterior side ; ])ostorior muscular impres- 

 sion faint, large in area, almost indistinguishahlc ; posterior 

 margin truncate in normal specimens. Surface marked hy 

 strong concentric rugose lines. 



The generic position of this form is not satisfactory, hecause 

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

 Cyprina, although sometimes similar forms are referred to that 

 genus l)y paleontologists in Europe ; neither is it a Cardium as 

 described by Conrad. 



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

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

 the thousands throughout the vertical and geographic extent in 

 Texas and Arkansas, beginning in the lowest fossiliferons hori- 

 zon and extending to the top. 



A few specimens were found about ten miles west of Cllen 

 Rose, upon which fragments of the thick calcified shell structure 

 were ])reserved, showing it to have a concentrically striated sur- 

 face and the anterior cardinal margin and l)caks to be as in Iso- 

 cnrdia, 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 ])late ii, fig. 5. 



Natica (?) texana Cc^nvad. 



U. 8. and Mexican Roundavy tSurvey, P- 1'57. plate xiii, figs. 

 In, h. 



This species is of rare occurrence in the l>eds at (den Rose. 

 It is not a Nalira, but inasmuch as only casts are i)reserved its 

 generic position cannot otherwise be stated. 



Viviparus (Natica ?) cossatotensis Hill. 



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

 lo(^, plate iii, figs. 4, 4o, 5, 5a. 



This form was originally described in my Arkansas Report 

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

 Natirn, but it is too imj)erfect to assert its generic position witli 

 certainty. The writer inclines to the l)elief tliat it is a fresh or 

 brackish water form. 



1 



