260 bulletin: museum of comparative zoology. 



well exposed at Kirkfield, which locality has become famous for its 

 beautiful crinoids, cystids, and starfishes. Though separated at this 

 locality by about twenty-five feet of strata (Rockland formation) 

 containing a good fauna in which a number of Trenton genera are 

 introduced (Platystrophia, Triplecia, Calymene, etc.), the Hull 

 contains a number of fossils which have survived from the Black 

 River. It is very near the horizon of the Curdsville. At Ottawa 

 this formation contains many echinoderms but very few Pleuro- 

 cystites, these fossils being abundant in a zone seventy-five feet higher 

 in the section. It was from these higher beds that Pleurocystites 

 squamosus and the other species described by Billings were obtained. 

 From this upper bed all the Black River species are absent and the 

 cystids are associated with species of the Clitambonites fauna. In 

 this same section at Ottawa there is a third zone of echinoderms, some 

 forty feet above the middle one, and in the Picton formation. This 

 zone has produced a number of cystids and blastoids associated with 

 Strophomena trilobata, Rafinesquina dcltoidea, and Cyclospira bisulcata, 

 and though, so far as I know, no Pleurocystites have been found at 

 this horizon, it is probably the horizon with which the Minnesota 

 cystid bed is to be correlated. In other words, in the Ottawa section 

 there is a lower, a middle, and an upper Trenton Curdsville bed, no 

 one of which is exactly of the age of the Curdsville bed of Kentucky, 

 and affording plain evidence that the echinoderms in themselves are 

 of no value in determining correlations. From a study of the asso- 

 ciates of the fossils it is very evident that the Curdsville of Kentucky 

 is most nearly of the same age as the Hull of Ontario, whence it follows 

 that the higher beds in Ontario are younger. The Minnesota " Curds- 

 ville" is youngest of all, and to be correlated with the Picton of 

 Ottawa and northern New York, and that is in turn younger than the 

 strata exposed at Trenton Falls. If on the other hand, the Curds- 

 ville of Kentucky is to be correlated with the " Curdsville" of Minne- 

 sota, then the base of the Kentucky section is above anything seen at 

 Trenton Falls, and not below it. 



Pleurocystites also appears in eastern New York and at Montreal, 

 this time in the Glens Falls formation, near the base of the Trenton. 

 These strata are probably of about the same age as the Hull beds of 

 Ontario and the Curdsville of Kentucky though no strict comparison 

 of the faunas is possible. 



The second point to be considered in the correlation of the Minne- 

 sota and Kentucky sections is the disposition to be made of the group 

 of Fusispiras so characteristic of the Fusispira bed. 



