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SMITHSONIAN INSTITUTION 



same bed, but there is good reason for believing that they all come from one limited locality. 

 The original of Hall's figure 15 (20th Report, pi. 11), is said in the description to be from 

 Bridgeport, Illinois. I have carefully examined this specimen, which is refigured herein 

 (PL XXXIV, fig. 7). It is of the same material and preservation as the other Bridgeport 

 specimens, and there is no authentic record of any Ichthyocrinus having been found else- 

 where in the Chicago- Wisconsin area ; so that I have no doubt it came from there. The 

 relative dimensions and prominence of the radial ridges of this specimen clearly indicate its 

 identity or close relationship with those of /. subangularis from the Niagaran shales at 

 Waldron, Indiana ; and the same characters show with equal force the distinctness of Hall's 

 Bridgeport specimen from the other form common in that region. 



In the way the collections have necessarily been made in the Chicago- Wisconsin region 

 we have little knowledge of the exact beds from which the fossils have been derived. The 

 dolomites of that area are held to be equivalent to the Lockport limestone, and to occupy a 

 position intermediate between the Waldron and Rochester shales and the Brownsport lime- 

 stone of the Tennessee area. We find from the Chicago region an assemblage of forms some 

 of which are more or less restricted to particular horizons in the other regions, and also some 

 not occurring elsewhere. Thus we have Ampheristocrinus, Lyriocrinus, Macrostylocrinus 

 and Periechocrinus, which are mostly confined to the shales both at Waldron and Rochester ; 

 Callicrinns, Lampterocrinus, Pycnosaccus, Zophocrinus, which are found in the limestones 

 of the other regions, but not in the Waldron or Rochester shales. And we also have genera 

 like Ichthyocrinus, Lecanocrinus, Eucalyptocrinus, and Myelodactylus, which occur in both 

 shales and limestones of the other regions. Hence the stratigraphy does not make for or 

 against the probable occurrence of the same species in the two regions. 



That the form of Ichthyocrinus occurring in the Niagaran of Northern Illinois repre- 

 sented by the well-known internal casts with usually distorted base, and to which Winchell 

 and Marcy with however incorrect a description gave the name /. corbis, is an absolutely 

 distinct species from that occurring in the Niagaran shales at Waldron and described by 

 Hall as I. subangularis, may be conclusively demonstrated. They are in fact far more readily 

 distinguished than some others which are separated by continental distances, for they fall 

 under different divisions of the genus by some of its strongest characters. /. corbis is a 

 rounded ovoid, with convex sides and circular cross-section, small basals, low and wide 

 brachials having a very rapid increase in width upward, and a consequent broad spreading 

 of the calyx within the first few ranges of plates. /. subangularis is elongate, irregular, with 

 strongly prominent radial lines, and subangular cross-section ; has large basals, relatively 

 longer brachials, and a much less rapid spread of the calyx from the base up. 



Actual measurement of a number of specimens is a more reliable guide in the matter of 

 relative proportions than general impressions, and I have applied such a test in this case. The 

 average of 15 representative specimens of /. corbis of all sizes, and 6 specimens of /. sub- 

 angularis from Waldron, including Hall's types, gives the following relative dimensions ; 

 I have added those of Hall's Bridgeport specimen of the latter species and of a cast in my 

 own collection, also from Bridgeport, in the same condition in which /. corbis is found : 



" 



Height to width 



Expansion of calyx 

 from RR to 1 1 Br.: 



at IIBr 4 at IAx 



of IBr t 





i : 1.6 

 1 : 1 

 1 : 1 

 1 : 1 



1:1.4 

 1 : 1 

 1 : 1 

 1 : 1.07 



1:4.4 

 1 : 2.4 

 1 :2 

 1:2.3 



i :2 

 1:1.7 

 1:1.5 

 1:1.6 



/ subangularis, 6 specimens 



Hall's Bridgeport spm. XXXIV, 7. . 

 New Bridgeport spm. XXXIV, 8. . . 



