On a new Genus of Graptolites. 209 



XXXV. — On a new Genus and some new Species of Grapto- 

 lites from the Skiddaw Slates. By II. Alleyne NICHOL- 

 SON, M.D., D.Sc, F.R.S.E., Professor of Natural History 

 in the University of St. Andrews. 



[Plate VII.] 



In the following communication I wish to draw attention to 

 some remarkable Graptolites collected by Mr. W. K. Dover 

 from the Skiddaw Slates, and placed by him in my hands 

 for examination. One of these is the type of a new and very 

 singular genus ; another is a large and tine new species of the 

 genus Thamnograptus ; and the third is a new species of 

 Didymograptus. 



Genus Azygogkaptus, Nicli. & Lapw.* 



Polypary simple, unilateral, consisting of a single mono- 

 prionidian stipe, which is developed from the central portion 

 of the sicula on one side. Cellules slightly overlapping. 



This genus completely fills up the great break between the 

 unilateral and bilateral siculate Graptolites, and seems to be 

 intermediate in its characters between the true Monograptidee 

 and the family of the Nemagraptidai. It agrees with the 

 former in the fact that the polypary consists of a single uni- 

 cellular stipe — but differs altogether in its mode of develop- 

 ment, the celluliferous stipe springing directly from one side 

 of the sicula about its centre. In this important character the 

 genus agrees with no other known Graptolites than Nema- 

 graptus, Emmons, and Camograptus, Hall, both of these, 

 however, including bilaterally developed forms. The cellules 

 of Azygograp>tus are essentially of the type of Monograptus 

 Nilssoni) Ban*., and thus link the genus indifferently to either 

 the Monograptidee or the Nemagraptidce. 



Subjoined is a description of the only known species of the 

 genus. 



1. Azygography Lapworthi, Nich. PI. VII. figs. 2-2c. 



Polypary simple, of a single slender monoprionidian stipe, 

 which takes origin from the centre of one side of a strongly 

 marked pointed " sicula." The length of the sicula is about 



* I have associated Mr. Lapworth with myself in the description of 

 this new genus, partly because its name was suggested by him, and 

 partly because it would not have been possible for me to satisfactorily 

 determine its position and affinities had it not been for his valuable and 

 elaborate researches on the development and classification of the Grapto- 

 lites (" An Improved Classification of the Rhabdophora," Geol. Mag. 

 vol. x. 1873). 



Ann. & Mag. N. Hist. Ser. 4. Vol. xvi. 1 9 



