39 2 



NEW YORK STATE MUSEUM 



ence becomes still more pronounced (in one I counted 36 thecae in the 

 same space) and the narrow long rhabdosome with its very closely arranged 

 thecae presents an aspect quite different from that of the typical form. 



These changes point evidently to racial degeneration preliminary to the 

 extinction of the species occurring at the end of the Champlainic. Since 

 they are also connected with areal separation, the forms should find recog- 

 nition as subspecies. 



The variety characteristic of our Utica shale I propose to term 



Glossograptus quadrimucronatus (Hall) var. approximatus now 



Plate 26, figures 10-15; plate 27, figures 6-7 



This is the form whose synrhabdosomes have been found by the author 

 near Dolgeville. It differs principally from the typical G. quadrimu- 

 cronatus in the dimensions (average length and width 

 of rhabdosome 40 mm and 2 mm respectively), the some- 

 what narrower sicular end and closer arrangement of 

 thecae ( 10-1 1 in 10 mm or 26-28 in 1 inch). The mucros 

 are less conspicuous than in the typical form but still 

 very distinct. The nemacaulus is thicker and frequently 

 of great length. 



Prof. Lapworth has in the description of Diplo- 

 Fi g . 337 Giossograp- g rapt us foliaceus in his manuscript report com- 



tus quadrimucronatus 



var. approximatus nov. men t e d on the fact that that species exhibits all varia- 



Portion of rhabaosome show- * I 



ing fillers in perisarc. x 5 . r i\ 1 r> • /"• 1 



(Comp. pi. 2 7 , fig. 6) tions irom U . d e n t a t u s- brongniart to G . q u a d- 



rimucronatu.s. He therefore apparently considers (">. quadrimu- 

 cronatus a derivative of I), foliaceus, a view which would seem to 

 be supported by this variety, for the latter could as well be regarded as a 

 variety of 1). foliaceus with paired apertural macros as one of G. 

 q u a d r i m u c r o natus. This is still more obviously true in regard to the 

 next variety, which in outline of rhabdosome and form ol thecae is nearest 

 related to D. foliaceus var. acutus, but furnished with paired thecal 

 spines that are even longer and more conspicuous than in the typical (>. 

 q u a d r i m u c r o natus. Using the presence ol these spines as the princi- 



