GRAPTOLITES OF NEW YORK, TART 2 347 



a Diplograptus fcliaceus var. incisus Lapworth 



Plate 24, figures 1-8 



G r ap t ol i t h u s prist is Hall (non Hisinger). Pal. N. Y. 1 <S 4 7 . v. r, pi. 72, fig. ir 



Description, Length of rhabdosome 25 mm to 45 mm ; maximum 

 width 3 mm, with parallel distal (antisicular) margins, but converging 



throughout its lower half to a pointed sicular extremity, the first two thecae 

 of which are provided with short lateral spines. Nemacaulus distinct, broad, 

 double, in antisicular direction prolonged to a distance eqtial to one fourth 

 of the length of the rhabdosome. Thecae 8 to 9 in the space of 10 mm 

 (20-22 to 1 inch), inclined at an angle of 45 to 50 with the nemacaulus; 

 ventral margin broadly concave, and pressed inwards deeply in the lower 

 half to form a deep excavation ; apertural margin horizontal or slightly 

 inclined ; occupying one third of the width of the rhabdosome and originat- 

 ing a broad pointed denticle. Surface smooth and unornamented. 



The characteristic features of this species are constituted by the 

 broadly convex edges of the thecae and the large and deeply concave aper- 

 tural margins, forming between them a broad triangular denticle. The 

 edge of each theca does not show the deep interspace so well seen in D . 

 angustifolius, the interspace itself being often reduced to a mere 

 oblique triangular slit with convex margins ; the apertural edge alone reach- 

 ing the outer boundary of the rhabdosome; the thecae appear to become 

 broadly sacculate in the upper third of their length, and the margin of the 

 aperture crosses a larger proportion of the periderm than in any other spe- 

 cies known to me. Judging from the various aspects presented by the 

 thecae, it would appear that the central part of each apertural margin is the 

 most prolonged, but except in the initial theca it is only usually produced 

 into a point which has a slightly ascending direction. The lateral depres- 

 sion of the sides of the apertural margins, gives a curious appearance to 

 certain compressed specimens ; a row of cavities or hollows running down the 

 rhabdosome midway between the axis and the general margin in the profile 

 view. This is most conspicuous in specimens preserved in slight relief. 



The affinities of this species are certainly with D . p r i s t i n i f o r m i s 

 Hall, from which it differs in general form, in width and the number 

 of thecae to the inch ; and also in the amount of inclination. Its relation- 

 ship with I), foliaceus var. acutus and other varieties is much 

 closer. 



Loc. y 2 mile below Little Mechin Point (415), Lower Canada and 

 Stockport, Columbia co., N. Y. 



Author's remarks. This is the most common of the New York Nor- 

 manskill forms. The collection of the State Museum obtained at Glenmont 

 near Albany contains synrhabdosomes [see pi. 24, fig. 5] and rhabdosomes 



