136 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 17, 



new Graptolites — a doubly and triply branched form which I termed 

 Dichograpsus, after ascertaining its identity with those remarkable 

 fossils discovered by Sir "W. Logan near Quebec, of which he sent 

 drawings to England many years back, and which, admirably en- 

 graved, were to be seen in the Canadian Department of the Great 

 Exhibition. 



There were also with these forms some foliaceous Graptolites, 

 which I have referred to Phyllograptus, Hall, together with the now 

 well-known species Graptolites Sagittarius and G. tenuis, the latter 

 very abundant. Mr. W. West, of Wimpole Street, has since made 

 some notable additions to this list, and some of his specimens are 

 figured on p. 137. 



Prof. Harkness has not only added to our correct knowledge of the 

 beds from which these are obtained, but has also furnished a few new 

 forms to the British list; among them, especially, the beautiful 

 Didymograpsus cadaceus, a swallow- tailed form, formerly described 

 by myself, from Canada. 



Indeed, the most interesting point about these collections is the 

 positive identity that appears to obtain between the fossils of the 

 Skiddaw slates and those of the Quebec group of Canada. In both 

 we have the ordinary genera of Lower Silurian Graptolites — 

 Graptolites, Diplograpsus, and Didymograpsus ; but mixed up with 

 these are some others peculiar to this horizon, viz. Phyllograptus t 

 the Dichograpsus mentioned above, and a kind of double Didymo- 

 grapsus, for which it is perhaps admissible to form a new genus — 

 Tetragrapsus. 



The series of forms in the Graptolite group is now so perfect, that 

 it may well be surmised that we have pretty nearly exhausted the 

 variations. 



There are simple Graptolites with a single row of cells, unbranched 

 or only irregularly proliferous — Graptolites. Straight Graptolites 

 with a double row of cells — Diplograpsus. Twin Graptolites, having 

 a single row of cells only on each branch, the bifurcation taking place 

 from the root — Didymograpsus (figs. 13 a to 13/) ; again, a form in 

 which this bifurcation takes place twice, the branches patent or 

 nearly close — Tetragrapsus (figs. Sa, 8b); Phyllograptus (Hall), also 

 composed of four stems, i. e. doubly branched, but the stems close 

 together, and giving the appearance of a cruciform Diplograpsus 

 (figs. 7a, 7 b). Tetragrapsus comes near to the Canadian genus Dicho- 

 grapsus, which is doubly branched, and again dichotomous more than 

 once, in most of the species. But the main character which distin- 

 guishes Dichograpsus is the presence of a corneous plate which en- 

 velopes all the lower part of the branches (fig. 10), and which is not 

 known in any other genus of the group ; it has not, indeed, been seen 

 in more than two or three species of Dichograpsus, but it may not 

 have been preserved in all cases. I still believe that the best ana- 

 logy for this plate is in the basal plate of Defrancia, a Bryozoon com- 

 pared with it several years back by Huxley. Lastly, we have the 

 extremely branched form Dendrograpsus (Hall), the name of which 

 sufficiently expresses its ramose character. The branches in this 



