666 NEW YORK STATE MUSEUM 



Tetragraptus (Etagraptus) lentus sp. nov. 



Plate 9, fig-ures 7-10 



Description. Sicula large (2.1mm) and slender. Primaiy theca generally 

 budding from the first third of the sicula, this and the next forming right 

 angles with it; narrow, tubular and long (1.0). Secondary branches at 

 first foiTning obtuse angles, generally about 120°, with the primary branches, 

 but later on assuming positions perpendicular to them. Branches narrow 

 and flexuous, maximal width .37 mm, total length not known. Thecae 

 extremely long (attaining a length of 2.4mm), tubular and slightly curved; 

 seven times as long as wide, little widening to^vard the aperture ; over- 

 lapping not more than one fourth of their length, numbering 6 in 10 mm. 

 Outer wall subparallel to the axis of the branch, forming an angle not 

 surpassing 5° with the latter. Apertural margin straight and perpendicular 

 to the axis of the branch. 



Position and localities. Graptolite bed 3 at the Deep kill. On slab with 

 Didymograptus bifid us and Goniograptus geometricus. 



Remarhs. There is no similar form known to the*writer which would 

 invite comparison. Coenograptus gracilis, while readily dis- 

 tinguished by the arrangement of the branches, has very similar thecae and 

 branches. The direction of the secondary branches is in this species the 

 same as in Tetragraptus a p p r o x i m a t u s Nicholson, which, however, 

 is entirely different in the character of sicula and thecae, 



DIDYMOGRAPTUS McCoy 



The genus Didymograptus was proposed by McCoy [1851, p.9] for 

 uniserial forms, which are bifid from the base. Later Hopkinson separated 

 the forms with solid axes in the branches under the generic term Dicello- 

 graptus. 



We have in the introduction attempted to trace the genetic relationship 

 of the species of Didymograptus, described in this publication, to forms 

 with a greater number of branches ; and there discussed the fact that the 

 various Didymograptidae represent the biramous development of different 



