654 



NEW YORK STATE MUSEUM 



While in general form, size and aspect the form closely agrees with the 



figures given of T. pendens, there appear some small differences in the 



measurements which are thought to be too insignificant to indicate a variety. 



The rhabdosome is small (14 mm greatest length observed), slender 



(5 . 5 mm greatest width observed), consisting of four dependent branches 



which are subparallel or approach slightly distally, form- 

 ing long, flat crescents. The sicula is relatively short 

 (1 . 3 mm) and rapidly widening. The thecae open 

 toward the inner side of the rhabdosome, increase only 

 slightly in width, and the branches are therefore of 

 nearl}^ uniform width. The thecae are long, slender 

 tubes (length 1 . 6-2 mm), which at the beginning of the 

 branches do not overlap more than ^ of their length, 

 the overlap increasing to not quite ^ of their length. 

 Eight to 10 thecae were counted within the space of 

 10 mm. The angle of inclination of the thecae increases 

 to 20° or even 22° when the branch assumes mature 

 features. The outer margins of the thecae are slightly 

 concave, with a small increase of the curvature near the 

 aperture. The apertures are straight or slightly con- 

 cave, and they form an angle of about 110° with the 

 axis of the branches. 



Position and localities. In the zone with D i d y - 

 mograptus bifidus and Phyllograptus anna, on slabs with 

 Didymograptus bifidus and Groniograptus geometric us. 

 In Great Britain it occurs in the Middle Skiddaw slates of the Lake district, 

 associated with Didymograptus (gibber ul us) caduceus and 

 Phyllograptus cf. typus. 



Remarks. It has been pointed out by the author of the species that it 

 is related to T. fruticosus, but readily distinguished from the latter 

 more common type by its much more slender form, the more uniform width 

 of the branches, closer arrangement of the thecae and smaller angle of inclina- 



Fig. 55 Tetragraptus 

 pendens Elles. Enlarge- 

 ment of proximal portion 

 of rhabdosome to show the 

 character of sicula and the- 

 cae. Deep kill. x4.5 



