164 SHIMER AND GRABAU — HAMILTON GROUP OF THEDFORD 



acteristic species. Spirifer sculptilis is not known outside of this bed in 

 the Eighteen-mile Creek region, and Tropidoleptus carinatus is best rep- 

 resented in this bed, as is also Rhipidomella vanuxemi and Rh. penelope. 

 In some localities Favosites billing si is extremely abundant, occurring as 

 heads often 2 feet or more in diameter. Heliophyllum confiuens is also 

 found here, as well as Craspedophyllum subcsespitosum. Among other 

 species restricted to this bed are : 



Pleurotomaria lucina Hall. Centronella impressa Hall. 



Plethomytilis oviformis (Conrad). Stropheodonta nacrea (Hall) {PJiolido- 



Goniophora modiomorphoides Grabau. strophia iowaensis) (Owen). 



Conocardium normale Hall. Spirifer granulosus (Conrad).* 

 Vitulina pustidosa Hall. 



A few specimens very similar to Sp. mucronatus var. arkonensis have 

 been found. 



The lower shales, 57 feet thick, are characterized by Spirifer mucronatus, 

 which in the lower beds is chiefly mucronate, but higher up becomes 

 more condensed laterally. With this species occurs the normal Hamil- 

 ton fauna of New York, which is most abundantly represented in the 

 lowest and the highest beds of this series. f 



In attempting to correlate the faunas of the two localities the question 

 of the synchrony of the Encrinal limestone of both at once arises. Cal- 

 careous beds and even beds of pure limestone are not unknown at vari- 

 ous levels in the Hamilton strata of western New York and Ontario. 

 We know of none, however, which have the thickness and lithic char- 

 acter possessed by the Encrinal limestone of western New York, which 

 has been traced over a wide area. At Thedford the limestone thus 

 denominated has a similar thickness and lithic composition and texture, 

 and there is no other limestone bed with which it can be compared. 

 There are other beds of limestone, consisting, like the Encrinal, of com- 

 minuted crinoid remains, but they are always quite thin. The compo- 

 sition of the fauna of the Thedford Encrinal bed, so far as it has been 

 ascertained, is, though meager, closely comparable to that of the Encrinal 

 of western New York. In both regions Spirifer sculptilis is characteristic 

 of it and is apparently restricted to it. The other species found in the 

 Encrinal limestone of Thedford are, with the exception of Favosites turbi- 

 nate characteristic of the Encrinal limestone of Eighteen-mile creek. 

 None of them have been found in any of the other calcareous layers, 

 where, had they existed in this region at the time of their deposition, 

 they ought naturally to have occurred. Thus we believe it is not a true 



* For a complete account of these faunas, see Grabau, 1898. 

 f Grabau, 1898. 



