GRAPTOLITES OF NEW YORK, PART 2 467 



not only in the exterior of the rhabdosome, but with which it lias also in 

 common the character of the skeleton, as far as this is known in the latter 

 species. 



Hall's type came from the Normanskill shale at Normanskill. Gur- 

 ley's fine material was collected in the same horizon at Stockport, Columbia 

 co., in this State, and I have before me suites of specimens from Schodack 

 Landing and Chatham in Columbia count)'. The species is nowhere com- 

 mon and frequently found fragmentary. Weller has also observed it in 

 the Normanskill shale of New Jersey and T. S. Hall has figured as 

 Clathrograptus cf. geinitzianus a fragment from Lawson, New 

 South Wales, which may be either conspecific or belong to a closely related 

 species. 



retiolites Barrande 



The Siluric of North America has thus far afforded but a single 

 diprionidian form. This belongs to the genus Retiolites the last of the 

 graptolites with a double series of thecae. Retiolites is the longest known 

 and most typical of that group of genera (Retiograptus, Stomatograptus, 

 Gothograptus) which combine the extreme reduction of the periderm to 

 little more than a network of conchiolinous fibers with their position at the 

 end of the biserial or diprionidian graptolites. 



The significance of this combination in regard to the probable mode of 

 life of these graptolites has been discussed in the first part of this work 

 [Mem. 7, p. 5 1 8] and the suggestion made that with these forms the tend- 

 ency to lighten the perisarc evolved by the floating or actively swimming- 

 habit of the later graptolites had reached its climax, and as the dominance 

 of some of the species in the middle Siluric indicates, also produced a 

 number of well adapted forms. 



That the network, however, was not the only perisarcal layer, can 

 already be surmised from the fine carbonaceous film that covers parts of the 

 compressed specimens [see pi. 31, fig. 6] ; and the fact that pyritized speci- 

 mens [see pi. 29, fig. 7] are totally smooth on their surface, demonstrates 

 that there was still an outer continuous layer. In fact, Holm has shown on 



