308 NEW YORK STATE MUSEUM 



fourth their length ; with obliquely directed strongly convex outer wall, 

 which on its highest mesial part carries a spine; and slightly introverted or 

 introtorted apertural portion, which is inclosed in a semicylindric excava- 

 tion, occupying one third the width of the rhabdosome. 



Position and localities. Hall knew the species only from the black 

 slates at the Normanskill (Kenwood). It has also been obtained in great 

 number and several varieties in the Normanskill shale at Glenmont, X. Y. 

 and Mt Moreno near Hudson and at numerous other localities (as Moor- 

 dener kill near Castleton, Poesten kill at Troy, neighborhood of Tomhan- 

 nock, Rensselaer co., Stockport, etc.). It can be considered as one of the 

 common and characteristic graptolites of the Normanskill shale in the slate 

 belt of New York. It is likewise common in the northern extension of this 

 belt through Vermont into Canada where Ami and Lapworth have repeat- 

 edly observed it. It has long been known in Great Britain and the mono- 

 graphers cite it as a very abundant fossil from man)' outcrops of the 

 Glenkiln shales in Wales, Scotland (Moffat and Girvan areas) and Ireland. 

 Linnarsson and Tullberg have recorded it from Sweden (zone of Coeno- 

 graptus gracilis) and T. S. Hall has recognized it in collections from 

 Victoria, Australia. 



Remarks. The characters of the earliest thecae ofD. sextans have 



been elaborated by Elles and Wood and described as follows : 



The earliest thecae, th. i 1 and th. i 2 , are of considerable size, and 

 unlike the corresponding thecae of most of the other Dicellograpti, only a 

 small fraction of their length assumes a horizontal direction, most of their 

 growth being obliquely upward and outward. The same is also the case 

 with th. 2 1 and th. 2 2 , which, however, develop alternately as in other spe- 

 cies. Hence the proximal end, owing to this alteration in the direction of 

 growth, approximates (like D. anceps) closely to what may be termed 

 the Diplograptid type. 



We figure here | 2 29 j the reverse aspect of a nepiastic growth stage 



which would seem to indicate even a short initial downward growth of one 



or both of the first thecae, similarly as in the Diplograptidae. The same 



organism shows the hairlike virgclla and the apertural position of the lateral 



spines, as well as the primitive form of the first thecae. 



