594 Systematic Paleoxtology — Upper Devonian 



Devonian deposits occur it has been regarded as indicating an upper 

 Devonian horizon. Its range tlirongh tliis division of the Devonian, 

 however, varies in different countries and sections. Where the life zones 

 are best defined, as for example in New York where this later stage of 

 Devonian (the horizon of EhynclionelJa cuhoides (Ciiboides zone = Tully 

 limestone)) is followed by that of ManUcoceras intumescens (Intumes- 

 cens zone = Naples beds) and this by the Chemung fauna with S. dis- 

 junctiis. Here its lower range is more restricted than in certain of the 

 sections in Devon, Germany and Russia, in which the species appears with 

 the initiatory upper Devonian fauna. In New York, on the other hand, 

 tlie species rises into strata wliich show a marked change in the faima 

 and the ingress of later, that is post-Devonian types, and it is assumed 

 that here at least the species continues beyond the migration or extinc- 

 tion of its normal faunal associates, and becomes thus what has been 

 termed a " superstitial " or surviving species. 



Spirifer disjunctus may be at once distinguislied from other Devonian 

 Spirifers of this age by the fine ribs on its fold and sinus. However 

 it may vary in its form and proportions from a short, winged, plump shell 

 to a narrow, slender form with, greatly extended cardinal angles, this 

 feature abides as its distinguishing character. Its associate S. mesas- 

 trialis has fine lines on fold and sinus and S. mesacostalis a single 

 more or less defined median rib in the sinus. No other Devonian species 

 approach it or are a source of confusion with it. 



It will be observed from the illustrations cited and here given how much 

 the shell varies in its aspect, and some of the expressions are peculiar 

 to the locality at which they are found, but Ihere is not enougii evidence 

 to show that any is confined to a given horizon or dissociated at any spot 

 with other expressions of the species. All attempts, therefore, that have 

 been made to ascribe a zonal value to these various expressions of the 

 species have been attended with little success, and the various designations 

 at one time applied by Hall to such expressions have been for the most 

 part withdrawn from usage at the instance of that author himself. 



The species is of relatively rare occurrence in the lower beds of the 

 Chemung of New York, and in the New York succession culminates after 



