120 BRITISH GRAPTOLITES. 



The origin of the two main stipes from the central part of the sicnla gives a 

 characteristic appearance to this genus, the persistent sicnla itself always resembling 

 a central " radicular bar." 



Another characteristic feature is constituted by the regularity of the secondary 

 branching as contrasted with the general irregularity of the branching in the genera 

 already described. 



The earliest form recognised as belonging to this genus was the Graptolithus 

 gracilis of Hall, figured and described by him in his ' Palaeontology of New York,' 

 1847. The same form was afterwards described and figured by Emmons in his 

 ' American Geology,' 1855, and made the type of a new genus — Nemagrapsus 

 (Nemagrapsus elegans). Although Emmons's figure and description are somewhat 

 indifferent, yet it is clear both from his figures, and from the circumstance that the 

 associates of his A r . elegans are those of Or. gracilis, that the two are the same. 



In 1865 Hall himself, in his ' Graptolites of the Quebec Group,' p. 43, wrote 

 that Emmons's typical species of Nemagraptus (elegans) is " apparently a part of an 

 individual of Graptolithus gracilis, or some similar species." Geinitz, however, in 

 his review of Hairs work (1866), proposed the name of Stephanograptus for 

 this form, and in 1868 Nicholson suggested the title of Helicograpsus. In the same 

 year Hall, ignoring Emmons's earlier generic name, proposed the name Cceno- 

 graptus. 



The titles Stephanograptus and Helicograptus are both of older date than 

 Ccenogi'aptus, but Nemagraptus has the priority over both. We are compelled, 

 therefore, to employ Emmons's name of Nemagraptus for this genus, but having 

 regard to the fact that Ccenograptus has been in general use for thirty years, and 

 has entered into geological nomenclature as the name of a special geological 

 horizon, we have placed it in brackets after Emmons's designation for the sake of 

 reference and convenience. 



The Nemagrapti may be classified into the following two groups : 



Group I. — Type N. gracilis Group II. — Type N. explanatus. 



N. gracilis N. explanatus. 



var. surcularis var. yertenuis. 



var. remotus 

 var. nitidulus 



Group I. — Type N. gracilis. 



Nemagrapti in which the main stipes are usually curved in a sigmoid fashion 

 and give off numerous secondary branches. 



