92 BEITISH CtRAPTOTJTES. 



unknown, and np to the present time it lias only been recorded from tlie Skiddaw 

 Slates. 'i'-\vo specimens only are known to ns ; both of these belonged originally to 

 Nicholson, and are now in the Natnral History Mnseum, Sonth Kensington. 



NOTE. 



There remain two so-called genera to be descril)cd in tliis connection. These 

 are Azygograjptus of Lapworth, and Phyllograptus of Hall. 



Azygograptus has wholly lost the power of dichotomy, so characteristic of the 

 typical members of the Dichograptid family ; and the genns inchides within it, as 

 at present understood, a form showing characters approximatir.g in some respects 

 to those of the Leptograptidte. 



Phijllograpfiis, while fully retaining the thecal characters of the Dichograptidse, 

 and their power of dichotomy, has acquired more or less the outward Itabit of 

 Dqdograptu.s. 



Genus AZYGOGRAPTUS, Nicholsoii and Lapworth. 



1875. Azygograptus, Nicholson, Ann. Mag. Nat. Hist. [4], vol. xvi, p. 2G9. 

 1898. Azygograptus, Elles, Quart. Journ. Geol. Soc, vol. liv, p. 153. 



Polyimry simple, unilateral, consisting of a single stipe originating from the 



sicula at various levels, and bearing thecfe on one side only. 

 Thecx distant, overlap small, inclination uniformly low. 



The original description of this genus was founded on the characters of 

 Az. Lapivorthi, and it was regarded as belonging to the family Leptograptidse 

 (Nemagraptidae). But abundant examples of Az. Lapnrorflii subsequently obtained, 

 show that this species at any rate possesses the ordinary characters of some of the 

 slender forms which are grouped with the Dichograptidce (for example, Diclymo. 

 graciliH), and thus the type species in this genus is consequently best regarded as a 

 Dichograptid in which the power of dichotomy is altogether lost. This conclusion 

 wodld in the meantime carry with it the systematic position of the entire genus 

 as at present understood. Two out of the three other known species of the genus 

 present similar characters; but the third (Az. coelehs) shows in certain particulars an 

 approach to the Leptograptidae, and might perhaps be regarded as a transitional 

 form between the two families. 



Holm has described a form (' Geol. F(")ren. Forhandl.,' Bd. xvii, H. 3, p. 839) 

 which he believes is intermediate between Didymograpt'm and Azygograptn.'^. In a 

 normal Dldymograptiis, as already described, there are present ^^jiwimary stipe and a 

 Hecond stipe, communicating with each by means of a crossing canal ; in Holm's 

 intermediate form there is a primary stipe and a crossing canal, but no second stipe 

 is developed : the foi-ni is, in fact, an ahoiicd Didymograptn.^. 01)vious]y, there- 



