CHOCTAW AND GRAYSON TERRANES OF THE ARIETINA. 47 



The relationships of the two faun;\?, as far as now known, 

 may be seen in the following table: 



TABLE OF OCCURRENCE OF FOSSILS IN THE TERRANES OF THE 

 MAIN-STREET ARIETINA. 



Diplopodia texanuvi 



Hemipedina charltoni. . . 



Oi'thojysis sjj 



Cy])hosoma volanuni 



Holectypus charltoni. . . . 



Holaster completus 



Holaster nanus 



Holaster s^ipernus 



Enallaster it) flatus 



Enallaster texanus 



E piaster hemiasterinus. . 

 Terebratella wacoensis. . . 



Ostrea fra nklini 



Ostrea perversa 



Ostrea quadriiplicata . . . . 



Ostrea subovata 



Gryplicea mucronata .... 



Exogyra arietina , x 



Exogyra drake i 



Plicatula dentonensis. . . 

 Lima toacoensis 



Vela texana x 



Avicula dispar 



Modiola pedernalis 



Cucullwa recedens 



Trigonia clavigera ( 9) . . 



Astarte robbinsi 



Cardiuia sp 



" {Protocard.)texanum 

 Roudairia denisonensis . . 



Tapes dentonensis 



Cyprimeria texana 



Pholadomya ragsdalei. . 

 Pholadomya sancti-sabo' 



Homomya uuishita 



Pachymya austinensis. . 

 itella denisonensis. . 

 seriatim ^ diulata 

 Natiiilus texanus. , . . . , 

 Turrilites brazosensis — 

 Sphenodiscus pedernalis . 

 Hoplites texanus 



Ti^rit 



From this table it will be noted that, of forty-two species 

 found in the two terranes, only ten, or less than one-fourth 

 part, are common to them both, and that, of the eleven species 

 of sea-urchins, not a single species is known to occur in both. 

 The absence of Cephalopoda in the Choctaw terrane is also 

 noteworthy. Of Tiirritella denisonensis, common in the 

 Choctaw, the Grayson has thus far yielded only a single 

 example. The Gryphaia mucronata is not known in the 

 Choctaw, nor does Ostrea subovata seem to extend up into 

 the Grayson terrane. The latter observation essentially ap- 

 plies also to the association of Terebratella wacoensis with 

 Exogyra arietina, althcjugh, as above noted, a single speci- 

 men of the Terebratella has been found by Hill in the Gray- 



