684 NEW YORK STATE MUSEUM 



have ventured to describe it as belonging to a different species. The fact how- 

 ever, that this specimen occurs in a zone where Bryograptus lap- 

 worthi has become extinct, and that it agrees still more closely with a 



species of Didymograptus occurring in the same 

 zone in Great Britain, justifies the identitica- 



L!^:|w:'ffiS^*nrln%"h*'S *io^- Fr^^ Bryograptus lapworthi 



the specimen differs by the somewhat closer 

 arrangement of the thecae, the more prominent denticulations of the apertural 

 margins and the greater angle of divergence of the branches. 



Didymograptus cuspidatus sp. nov, 



Plate 13, figure 16 



Description. The single specimen of this species observed presents the 

 following characters. Primary disk and nema not present. Sicula short, 

 apparently not longer than . 8 mm, and inconspicuous. Branches originate 

 suborally, at slightly different levels; diverge at first at an angle of 140°, and 

 become later subhorizontal. Length of mature branches has not been 

 observed ; their width is at first very small (.4 mm), but increases rapidly to 

 the double dimension. Thecae number 11 to 12 in 10 mm, are slender (three 

 times as long as wide in the proximal portion 

 of the branch), but rapidly become wider, their 

 apertural width being in later thecae one half the 

 length; they are hardly in contact in the earlv piaatus sp.^ no vfproximaY^ portion 



^ ■ "Of rhabdosome. Ash hill quarry at 



portion of the branch and overlap later to about ^o"°t Moreno. x5 

 one thii'd to one half of their length. The inclination of the early 

 thecae is only about 18°; it increases to about 40° in the apertural part of 

 the later thecae. The latter is characteristically prominent, giving the thecae 

 a cuspidate appearance ; the outer margin is straight in the proximal portion 

 of the thecae and concave in the apertural ; the apertural margin slighth' 

 convex in the outer and concave in the inner part ; on the whole appearing 

 straight and vertical on the axis of the thecae. 



