GRAPTOLITES OF NEW YORK, PART 2 349 



nemacaulus in compressed specimens oi I). foliaceus. The thin- 

 ness ol the nemacaulus in our material is especially noticeable in the numer- 

 ous specimens from Glenmont, in which it is coupled with extreme length 

 [see pi. 24, fig. 1 |, making the feature the; more striking. Since Lapworth in 

 his description of 1 ) . foliaceus var. i n c i s u s expressly mentions the 

 broad character of the nemacaulus, it is possible that these Glenmont forms 

 again represent a different variation, which is also suggested by the fact that 

 many of the rhabdosomes possess as many as 10 thecae in 10 mm. On the 

 other hand the rhabdosome and thecae would not seem to differ sufficiently 

 in shape to warrant a differentiation. 



The sicular end though mostly protected only by the two short lateral 

 spines, will sometimes possess a longer (4 mm), stiaight virgella [see text 

 fig. 286] and a minute second sicular spine. 



In the largest rhabdosomes the maximum width increases to 4 mm and 

 the number of thecae in 10 mm sinks to 8 (20 to the inch). In others again, 

 which still present the characters of this variety, the number of thecae rises 

 to almost 10 in 10 mm (24 to the inch). The figures for the thecae should 

 therefore be probably extended from 8 to 10 (20-24 to the inch instead of 

 20—22 ). 



b Diplograptus foliaceus Murchison var. acutus Lapworth 



Plate 25, figures i, 2 



Graptolithus pristis Hall (? Hisinger). Pal. N. Y. 1847. v. 1, pi. 72, fig. 10, ip 



Description. Rhabdosome from 25 to 40 mm in length, with a maxi- 

 mum diameter of 3 mm ; margins parallel for the final three fourths of their 

 length ; converging slowly towards the sicular end which is furnished with 

 a central and two short lateral spines; about 1.25 to 2.50 mm in length. 

 Thecae 11 to 12 in 10 mm (28-30 to 1 inch), inclined at an angle of about 

 50° (40 and over), overlapping for about one third of their length ; the free 

 ventral margin crushed inwards below to form a well marked excavation, 

 and convex above, the edge becoming finally perpendicular. Apertural 

 margins of each theca horizontal, and usually slightly concave, the denticle 

 which is often bordered by a slight ridge being slightly introverted, and in 

 flattened specimens, prolonged into a short and slightly inclined spine. 



The most peculiar characteristics of this form are (1) the number and 

 form of the thecae, their frequently convex apertural edge, with its project- 

 ing, oblique denticle or short horizontal spine. 



