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NEW YORK STATE MUSEUM 



same horizon other genera and species are in a most flourishing condition. 

 The restraint must, therefore, have been intrinsic and I believe, we can not 

 err in concluding that the vital power of these races had failed and the 

 deficiency of vital power had first led to the appearance of these pathologic 

 looking mutations and finally to the extinction of the races. We have hence 

 here cases of racial old age and of spinosity as an expression of waning 

 vital power. 



As a third group of spines in graptolites, we have to consider those 

 which appear on other than the most exposed places and on the rhabdo- 

 somes of forms clearly not astophylogerontic. The best examples known 

 to me are Tetragraptus acanthonotus Gurley, Didymograp- 

 tus spinosus Ruedemann and the genus Glossograptus. The first 

 and second of these occupy unique positions within their respective 

 "genera" by differing from all other congeners in possessing besides " acute 

 denticles," or apertural mucronal extensions, spines all along the dorsal side 

 of the branches [see Mem. 7, text fig. 85]. In Tetragr. acanthonotus 

 these spines are so distributed that they correspond to every second theca 

 and in Did. spinosus one is placed opposite each theca; and in 

 Glossograptus not only the apertural margins are provided with spines on 

 either side, but there are also series of spines running down the middle of 

 the lateral sides, the latter corresponding to the dorsal spines of the two 

 other species mentioned. There is nothing in Tetragr. acanthono- 

 tus to indicate that it could have been a phylogerontic form, it appears 

 with the earliest Tetragrapti in the lowest Quebec zone and is ruggedly 

 developed. It is somewhat different with D i dy mograp t u s spinosus, 

 which appears after the culmination of the genus Didymograptus and by its 

 small size and somewhat rapid expansion has a contracted aspect, suggestive 

 of a phylogerontic condition. Yet for another reason we will discuss it in 

 this connection. Glossograptus appears fully armed with spines in the 

 third Deepkill zone together with the first Diplograpti and continues in 

 this condition through the next formations to its extinction. This is 

 clearly not a case of phyloparaplasis, for the genus gives by the number of 



