lxx BRITISH CxRAPTOLITES. 



partitions are attached to the solid axis, and that the only way in which 

 communication could take place was by assuming that the cell partitions are 

 triangular plates, their apices attached to the axis, having " an unequally arched 

 or convex upper surface, and a concave lower surface." Nicholson states 

 that in those examples of Glimacograpsus studied by himself there exists a 

 distinct common canal, and the figures of the various " aspects " given by 

 him illustrate well his views of the structure of the genus. The term " suture," 

 which was in this connection here first suggested by Nicholson for the median 

 groove or line formed at the surface by the septum, has subsequently been 

 generally adopted. 



He discusses the true character of Linnaeus' Gr. scalaris, and gives a brief 

 historical sketch of the species. He is inclined to think that it is not a Grraptolite 

 at all, " at any rate it is impossible to say whether it is the scalariform impression 

 of a mono-prionidian or di-prionidian form." 



He describes four species in addition to (1) C. teretius cuius, viz. (2) G. innotatus, 

 (3) G. tuberculatus, (4) G. antennarius, and (5) G. bicornis. His figures of this 

 last named species include both the peltifer and tridentatus varieties of later 

 authors. He states that he has " little or no hesitation in comparing the basal disc 

 or cup in G. bicornis with the disc of Dichogra/psus" &c. 



In 1870 Hopkinson published a paper on the genus 



jt ( , • ' Dicranograptus, Hall. He regards Dicranograptus as a 



" On the Structure distinct genus (not a sub-genus as Hall believed), and agrees 



and Affinities of the with Carruthers in restricting it to those Grraptolite forms in 



GemisDicranograptus," which the proximal portion is di-prionidian and the distal mono- 



ag., vo . vn. prionidian. He differs, however, from Carruthers in believing 



it to be more nearly allied to Glimacograptus than to Diplograptus. Although 



he describes the proximal extremity of the polypary as " composed of two series of 



thecge, each having its own common canal," he somewhat modifies this assertion by 



saying in a footnote that " I am by no means certain that the two series are 



thus isolated." " Glimacograptus and Dicranograptus alike differ from Diplograptus 



in the fact that the separation of the hydrothecas is only occasionally seen, and very 



seldom extends to the common periderm, and their apertures are in a hollow which 



appears to be excavated out of the polypary. Dicranograptus only differs from 



Glimacograptus in that its thecae are usually, but by no means always, more or less 



prolonged distally." 



Hopkinson describes and figures the following species in this paper : (1) 

 D. ramosus, (2) D. Clingani, (3) Dicranog. sextans (doubtfully referred to this 

 genus), (4) D. Nicholsoni (the web which seems to unite the branches for a short 

 distance after bifurcation, he suggests, may be possibly analagous to the central 

 disc of Dichograptus), and (5) D. formosus. 



