GRAPTOLITES OF NEW YORK, PART I 703 



theca each and have a length of 1 . 8 mm each. The theeae are extremely 

 slender, tubular, -without any noticeable widening toward the aperture, ovei- 

 lapping about one third of their length ; diverging fi-ora the axis of the branch 

 not more than 12°; their outer walls and apertural margins are straight, the 

 latter half as wide as the branch and forming an apertural angle of 5°. The 

 brachial theeae number 8 in 10 mm. 



Position and locality. Graptolite bed 3 of the Deep kill section, belong- 

 ing to the zone with Didymograptus bifidus and Phyllograptus 

 anna. 



Remarlcs. The principal stems are found to be composed of theeae formed 

 by the successive bifurcations. As alternately the right and the left of the 

 two diverging theeae become internodes of the stem, the latter shows still an 

 obscure zigzag line, suggestive of an origin identical with that of the principal 

 stems of Goniograptus. While the mode of branching of Sigmagi'aptus, like 

 that of the younger coenograptids, has to be designated as monopodial or 

 lateral, one of the branches always essentially retaining the direction of the 

 mother theca, still the faint presence of a zigzag curve in the principal stem 

 indicates that the mode of branching in this form is also originally dichotomous 

 in character, and differs only from that of Goniograptus in the greatei' degree 

 of the divergence of the theeae, from which the denticulate branches originate, 

 and a corresponding lesser degree of divergence of the stolonal ov stem theeae. 



No similar form, which would invite comparison, is known to the writer. 

 Coenograptus gracilis, while easily distinguished by the arrangement 

 of the branches, has very similar theeae and branches. 



Family f-hyllograf'tidae Lapworth 



PHYLLOGRAPTUS Hall. 1857 



This genus was first defined by Hall in the report on Canadian graptolites 

 [1857, p.31] and more full}- described in his Gi-aptoUtes of the Qxhehec Group 

 [1865, p.ll8]. Hall recognized thus early the essential facts of its structure, 

 viz the composition of the rhabdosorae of four branches, which have coalesced 



