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STRATIGRAPHIC TAXONOMY 663 



belong to the new genus mentioned in the foot-note on page 669 as bear- 

 ing a general resemblance to the Ordovician DelmaneUa suhcequata, but 

 differing in having a perforated deltidium like Clitambonites. Then 

 there are three species of amother genus — perhaps Eostrophomena Wal- 

 cott — ^that probably gave rise to the Strophomenidce. Next, there are 

 two species allied to Polytoechia, and finally a new type of SyntrophiidiE. 

 With these brachiopods, but more commonly in separate layers, are found 

 other fossils as follows: Ophileta sp., Eccyliopterus cf. triangulus, 

 Turritoma cf. acrea, Hormotoma cf. anna, very slender shells like 

 Murchisonia linearis Billings, Cameroceras sp., Leperditia sp., and small 

 ostracods suggesting Aparchites. 



Perhaps only very locally the third division is succeeded by about 250 

 feet of unfossiliferous, varicolored argillaceous limestone, and this finally 

 by a more widely but also not generally distributed bed of fairly pure 

 though somewhat argillaceous, thin-bedded limestone, sparingly fossil- 

 iferous, and reaching a maximum thickness of 400 to 500 feet. To- 

 gether these two beds constitute the fourth or uppermost division of the 

 Arbuckle limestone — at the same time also of the Canadian — in central 

 Oklahoma. It is followed by sandstones, shales, and limestones of the 

 Simpson formation, but the exact contact, which probably is uncon- 

 formable by overlap, has not been observed. 



As said, fossils are rather rare in the upper member of the fourth 

 division. iSTear the top Mr. Chester A. Keeds found a bed containing 

 good examples of Didymograptus, one of the two forms being identical 

 with the British species recently described by Elles and Wood as 

 Didymograptus artus. This graptolite horizon possibly is in the basal 

 Simpson, which formation doubtless extends farther down in the time 

 scale than the base of the Chazyan. Provisionally, however, it is in- 

 cluded in tlie upper Arbuckle. Its age, accordingly, would be late 

 Canadian. 



Compared with the tj^pical sections in the Canadian in the northern 

 Appalachian, Champlan, and Saint Lawrence Valleys, the faunas in the 

 Arbuckle limestone in Oklahoma indicate that we have here an unusually 

 well developed sequence of Canadian deposits. The presence of Ophileta 

 complanata near the middle of the 4,300-foot second division suggests 

 that from this zone on upward the Arbuckle limestone is not older than 

 the top of Brainerd and Seely's Division C in the Champlain section. 

 The underlying, unfossiliferous lower member of the middle division, 

 even though 2,000 feet in thickness, may then correspond to their Divis- 

 ion C. plus the older Tribes Hill-Stonehenge limestone which is at the 



