822 R. T. HILL — GEOLOGY OF KED RIVER. 



About midway on the vertical extent of the clays, fossils of the 

 peculiar Exogyra arietina of Roemer occur in the greatest abundance, 

 weathering out individually and in perfect jDreservation. To these shells, 

 especially the umbonal region, are attached small cubes of pyrite, which 

 quickly decompose, coating the shells with a thin layer of brown hema- 

 tite and probably converting the lime into numerous intercalated seams 

 of fibrous selenite, which abounds in the vicinity of the shell horizon. 

 Sometimes the shells are cemented into thin indurations of argillaceous 

 limestone, which at places makes a persistent band in the middle of 

 the clay bed. 



Above the zone of Exogyra arietina the clays are again somewhat 

 barren until near their summit, where they become slightly arenaceous 

 and contain impure limestone slabs bearing other fossils, most of which 

 also occur in the upper layers of the limestone beds underlying the 

 E. arietina clays. The E. arietina beds terminate upward with a horizon 

 of Grypheate oysters, which has been called ^' Gryphsea pitcheri,^^ Roemer, 

 "(r. navia,^' and "(7. pitcheri, var. navia,^^ in the writings of Roemer, 

 Shumard, White, the writer, and in all the reports of the Texas Survey. 

 This Gryphsea is none of them, however, but an entirely distinct species — 

 the Gryphsea mucronata of Gabb* — and I shall hereafter so designate 

 it. Gryphdsa mucronata is an especially characteristic fossil of the Exo- 

 gyra arietina beds, and I have found it in no other horizon. The lower 

 surface of the overlying Shoal creek limestone, which was deposited upon 

 the G. mucronata beds, is sometimes undercoated with this species (as 

 has been noted by Taif ),t but it clearly belongs with the clay beds. 



The fauna and flora found in the Exogyra arietina beds at Austin is as 

 follows : 



An iiiidescribed endogenous plant re- Lima sp. (&). 



sembling Eqidsetum (5)4 Plicatula sp. (6). 



Kingena ivacoeasis, Roemer (6) {d) (/). Slearnsis robbinsi, White (?>) ((?)• 



Diplopodia texanum, Roemer (e). Protocardia texana, Conrad [h). 



Exogyra arietina, Roemer (a). Pachyma austinensis, Shumard {b). 



Gryphsea mucronata Gabb (G. pitcheri, var. Dentalium sp. 



navia of various writers) (c). Tarritella, sp. (5). 

 Neithea texana, Roemer [b). 



Paleontologically the Exogyra arietina beds at Austin are liniited at 

 the top by the zones of Gryphsea mucronata, Gabb, and below by the 



*See Geological Survey of California: Paleontology, vol. 1, p. 274. 

 t Third An. Kept, of Texas Geol. Survey, pp. 347, 348. 

 J Species marked (a) ai'e in medial band. 



" " (b) are from indurated layers of upper part. 



" " (c) are from the top beds of the clay.s. 



" " id) only a single species found in these beds. 



'* ". (e) determined by W. B. Clark. 



" *' if) determined by Charles Schuchert. 



