494 NEW YORK STATE MUSEUM 



b Tetragraptus zone. This zone is represented at the Deep kill by- 

 two graptolite beds. While the fauna of one of the beds is largely composed 

 of species of Didymograptus, and in the other the representatives of the genus 

 Tetragraptus prevail, a comparison of the faunal lists of the two shows that all 

 species occurring in the first bed. are also found in the second, and that both 

 comprise one fauna. This consists of 32 species of graptolites. Of these, 

 16 are restricted to this horizon. Of the remaining, the greater majority 

 extend only into the lowest bed of the next horizon. 



The most important genus, which, both by the number of its species and 

 its individuals, gives to this zone its character, is the genus Tetragraptus, of 

 which eight species, out of the 10 of the Deep kill section, are either 

 restricted to this zone or clearly reach here the acme of their development. 

 The most stately and showy forms of the genus, as T. quadribrachiatus, 

 amii, fruticosus, serra and s i m i 1 i s, are those found in this bed. 

 The species of Tetragraptus found in the next horizon are either smaller 

 mutations of these species or dwarfed species with distinct paracmic features. 



Next in importance, in the number of individuals and species, is the genus 

 Didymograptus, which, however, is represented by only five of its 14 Deep 

 kill species in this zone, the others all being of later age. These five species 

 all belong, as a glance at the synoptic table of the genera [p. 508] will show, 

 either to the horizontal or to the declined series. The dependent (" tuning 

 fork ") and reclined forms are still entirely absent. The horizontal series of 

 Didymograptidae culminates in this zone and declines at its termination, 

 entering the next zone with only one species. 



The genus Phyllograptus is represented with three of its five Deep kill 

 species in this zone, but the largest and most typical form, P. t y p u s , is not 

 developed till the next zone is reached. 



The multiramous Dichograptidae are represented by Dichograptus 

 octobrachiatus, which here culminates ; and two of the three species of 

 the genus Goniograptus, attain here their greatest size and frequency. It is 

 however to be noted that the genus Clonograptus, so well represented in the 

 Main Point Levis zone, is absent entirely and only represented by its more 

 rigid and symmetric descendant, Goniograptus. It is, hence, very probable 

 that the Tetragraptus zone is preceded by a subzone characterized by the 



