G64 NKW YOUK STATE MUSEUM 



apeiiare and iucreasiug in overlap. Tlieir outer margins are strongly con- 

 cav'e and the apertiiral margins protracted, convex and recurving, giving the 

 distal thecae, in a lateral vie\v, a bluntly cuspidate aspect. The mature 

 thecae are thi-ee times as wide at the aperture as at the initial point ; the 

 overlap increases within each branch from one fourth to a little more than 

 one half. There are 12 thecae in 10 mm, the outer margins of which at the 

 beginning of the branch form an angle of 4° and at the distal parts an angle 

 of 40° to 60° with the axis of the branch. The angle between the apertiiral 

 margin and the axis of the branch is about 60°. 



Position and localities. The majority of the specimens have been 

 obtained in graptolite bed 2, of the Deep kill section, belonging to the 

 Tetragraptus zone ; a few were also observed in the beds with D i p I o - 

 graptus dentatus at Mt Moreno, near Hudson N. Y. The species 

 appears hence to range through several zones. 



Remarhs. This type is remarkable for the long slender shape of the 

 earlier thecae of the rhabdosome and the abrupt change to more curved and 

 moi-e closely arranged thecae, which are provided ^vith protracted apei-tui-es 

 and form a greater angle of di\'ergence with the axis of the branch. The 

 latter change is consequent on the closer arrangement and greater overlap <jf 

 the thecae. This abrupt change in the shape and arrangement of the thecae 

 appears as an interesting illustration of acceleration of development in the 

 ontogeny of the branches [1902, p.589], the transitional stages between the 

 earlier and mature thecae being here omitted. This feature, the small size of 

 the rhabdosomes and the gentle curvature of the branches will easily dis- 

 tinguish it from the other congeners. 



Tetragraptus pygmaeus sp. nov. 



Plate 12, flfirures 11-14 



Tetragraptus bigsbyi Ruedemann {pars). N. Y. State Paleontol. An. Rep't. 

 1902. p.590, fig.18 



In a former paper [1902, p.590] the author of the present memoir has 

 referred minute tetragraptids, occurring in considerable number in graptolite 



