CRAPTOLITES OF NEW YORK, PART I 703 



theoa each and have a length of 1 . 8 mm each. The thecae are extremely 

 slender, tubular, Avithout any noticeable widening toward the aperture, ovei' 

 lapping about one third of their length ; diverging from the axis of the branch 

 not more than 1 2" ; 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 thecae number 8 in 10 mm. 



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

 ing to the zone ^vith Didymograptus bifidus and Phyllograptus 

 anna. 



Reniarhs. The principal stems are found to be composed of thecae formed 

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

 two diverging thecae 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 Goriiograptus. 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 greater degree 

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

 and a corresponding lesser degree of divergence of the stolonal or stem thecae. 



No similar form, \\hich -would invite comparison, is known to the writer. 

 C o e n o g r a p t u s gracilis, while easily distinguished by the arrangement 

 of the branches, has very similar thecae and branches. 



Family f>hyllograf>tidae Lapworth 



PHYLLOGRAPTUS Hall. 1857 



This genus was first defined by Hall in the repoi't on Canadian graptolites 

 [1857, p.31] and more fully described in his Graptolites of the Quebec Group 

 [1865, p.118]. Hall recognized thus early the essential facts of its structure, 

 viz the composition of the i'ha1)d()some of four branches, \vhich have coalesced 



