504 P)'of. T. Sferry Hunt — On Cambrian and Silurian. 



polypary in these j)iizzling forms, afford evidence tliat in these 

 genera both series of thecee open into a sub-central common canal, 

 and that the various parts of the polypary are strengthened by 

 slender chitinous threads connected with the virgula. Nothing 

 further seems needed to show that these collectively form a very 

 distinct group from that already noticed. 



Among the species usually classed with Dijplograptiis are two 

 extraordinary types which seem to merit generic distinction. It is, 

 moreover, impossible at present to reconcile their general structure 

 with the previous interpi-etation of that of the siculate Biprionidm. 

 In the first of these types, which includes DipZo^rapirts spinulosus (H.) 

 and its allies, not only is each theca furnished with two long spines 

 or fibres, extending outwards from the angles of the aperture, but 

 the polypaiy itself is ornamented in addition with two opposite, 

 longitudinal rows of gigantic, isolated spurs, developed along the 

 median line of the periderm at right angles with the thecae. The 

 polypary in these forms is first visible as a small oval disc, bristling 

 with a host of incipient spurs, and is in its later stages, ornamented 

 with a greatly extended single (?) filiform virgula. The generic, 

 term Glossograptus was proposed for a form of this group by 

 Dr. Emmons in 1855, and although the designation can hardly be 

 considered appropriate now that the true form of the polypary is 

 known, yet it ought obviously to be retained. 



In the adult polypary of the species of the second type, a slender 

 spine is developed from the outer margin of each theca. At a small 

 but constant distance outside the general boundary of the polypary, 

 this spine subdivides into (or gives off) numerous minute threads, 

 which inosculate with those originating in a similar manner from 

 the spines immediately above and below. These threads form a 

 connected network or braid almost completely'- surrounding the 

 polypary. Perfect examples are excessively rare, but the specimens 

 belonging to the group can be recognized at once, their margins 

 being apparently furnished with minute tufts of tangled hairs. The 

 species of this type I propose to unite in a provisional genus under 

 the title of Lasiograptus (Gr. \daio<i, hairy, shaggy). 



The Analytical Table of the genera of Graptolites, embody- 

 ing the foregoing conclusions, together with the Appendix (already 

 referred to), containing descriptions of new genera, will appear in 

 the December Number. 



VI. — History of the Names Cambrian and Silurian in Geology.' 

 By Prof. T. Sterry Hunt, LL.D., F.R.S. 



{Continued from p. 461.) 



WE have already noticed that Barrande, as early as 1852, gave 

 the name of Primordial Silurian to the rocks which, in Bohemia, 

 were marked by the first fauna ; although he, at the same time, 

 recognized this as distinct from and older than the second fauna, 

 discovered in the Llandeilo rocks, which Murchison had declared to 

 ' Reprinted from the "Canadian Naturalist," new series, vol. vi. no. 3, p. 304. 



