CLONOGRAPTOS. 83 



Tlio llieciv are often well iji'cscrvcd, and ai-c o'ciicrally visible on tlic stiix's 

 of tlie tliii-(l, foui'th, and followino' oi-dci's, tliou^'li tlioy are not well seen on the 

 stipes of the earlier stages. 



As in other many-branclied forms, considerable atropliy frequently takes 

 place; thus, for example, in a typical CIonograpti(s all thirty-two stipes should 

 belong to the fifth order : 



8'^ -f 8'' + 8^ + 8'' = 132. 



But we may have forms in which the nnmber is only seventeen, which is 

 made n}) as follows : 



Cf + 9* + 2^ = 17; 

 and other aborted examples are known. 



Here, again, as in DicJior/yapfus, it is not the number of stipes which is the 

 essential characteristic of the genns, l)ut the degree of capacity for dichotomy. 



The only species that occiu's in Britain in a sufficiently satisfactory state for 

 specific description is G. tenellus, with its variety G. GaUavei. In these British 

 specimens the type of thecae is markedly similar to that characteristic of certain 

 Dicti/oiiema. The American examples of Glonograptus, however, originally figured 

 by Hall, present the ordinary type of cell seen in other Dichograptids. 



Glonograptus tenellus (Linnarsson). Plate XI, figs. 2 a — c. 



1871. Dichograptus tenellm, Linnarsson, Ofv. Kongl. Vet. Akad. Fc5rh. Stockhohn, vol. xxviii, No. 6, 



p. 795. . 



1876. Dichograptus tenellus, Linnarsson, Geol. Mag., dec. 2, vol. iii, p. 242. 



1876. TrichograjHus tenellus, Nicholson, Geol. Mag., dec. 2, vol. iii, p. 248. 



1880. Dichograptus tenellus, Linnarsson, Geol. Fiiren. Forhandl., vol. v, p. 132. 



1882. Dichograptus (?) tenellus, Brogger, Die Sil. Etagen 2 u. 3, p. 37. 



1885. Clonograptus tenellus, Herrmann, Die Graptolitenfam. Dichograptidse, p. 96. 



1892. Glonograptus tenellus, Moberg, Geol. Foren. Forhandl., vol. xiv, p. 89, pi. ii, figs. 1 — 3. 



1898. Clonograptus tenellus, Files, Quart. Journ. Geol. Soc, vol. liv, p. 474. 



Stipes somewhat rigid, very slender and thread-like, never exceeding '5 mm. 

 in width even at the apertnres of the thecse. Thecte nine to ten in 10 mm., 

 slender tnbes, widening slightly toward their apertures, inclined 20°, over- 

 lapping for the merest fraction of their length ; outer walls slightly ciirved, 

 three times as long as wide. Apertural margins concave, oblique, con- 

 spicuously mucronate. 

 Description. — Only a few specimens of G. tenellus have as yet been found in 

 Britain, but these are, as a rule, very well preserved. G. tenellus is characterised 

 by its extreme tenuity and by the mucronate thecal apertures, which recall strongly 

 those characteristic of many Didyonemns {D. cervicorne). This last character is, 



