STUDY OF THE GRAPTOLITES. 



185 



In the Genus Ptilograptus (Plate vi), wc have a somewhat analogous 

 mode of growth, but there is always an aspect or expression of the fossil 

 which distinguishes it from Thamnograptus. In one species of Ptilograp- 

 tus, moreover, cell-apertures have been detected on one side of the branches 

 or pinnulae. 



In the BuTHOGRAPTUS, we have a form bearing some analogies with the 



preceding ; but the rachis is flexible, and is not known to be branched, 



while the slender alternatinf^ pinnulae are flat and simple, as they have 



been observed in numerous individuals. The specimen represented is even 



less curved than the usual condition of this species in the slaty Trenton 



limestone of Wisconsin. 



Fig. 27. 



BuTHOGRAPTUs LAxus (enlarged). 



Still more obscure, and perhaps remote in its analogies with graptolites, 



is the Genus Inocaulis, consisting of flattened scabrous stems, associated 



with Dictyonema in the shale of the Niagara formation, which, from their 



carbonaceous substance and apparent graptolite texture, I have referred 



to the Graptolitidae.* 



Fig. 28. 



Inocaulis plumulosa. 



• The specimens of this fossil are extremely obscure, and the figure does not properly 

 represent its characters. It is not, however, a solid stem ; and though, as has been assorted, 

 it may not belong to the GRAPTOLiTiDiE, no other relation has yet been proved. 



[Assem. No. 239.] 24 



