GRAPTOLITES OF NEW YOItK. PART 1 



733 



one from the other. In fact, the two othei- foi-ms appear to represent 

 structures entirely different from that ofR. tentaculatiis. No forms 

 fitting into the generic diagnosis of Retiograptus have been found else- 

 where. The genus has, however, been emended l>y Freeh [1897, p.607], 

 who, asserting the similarity of its rhabdosomes with those of Gothogi-aptus 

 and Retiolites, sees the diagnostic charactei's of the genus in the lesser 

 development of the reticulate structure, which he infers is only suggested 

 by a row of hexagonal meshes along the margin, in the rectangular 

 arrangement of the thecae, the presence of spines and the resistibility of 

 the virgula. From the aspect of some specimens ofR. tentacxilatus 

 [pl.l6, fig.35] it is to be inferred that the entire periderm was reticulate in one 

 of its layers at least, and that the marginal meshes correspond to apertural 

 thickenings such as are also found in Retiolites. This would leave as dis- 

 tinguishing characters between the two genera only the rectangular arrange- 

 ment of the thecae and the presence of short apertural spines. 



If the thecae were placed perpendicular and the apertures straight and. 

 parallel to the axis of the rhabdosome, the genus, as represented by the 

 genotype here described would not be referable as a reticulate branch to 

 Climacograptus, where the thecae hold positions parallel to that axis and the 

 apertures fomi more or less transverse notches in the straight margin ; but 

 it should, in the writer's opinion, be considered to represent an extreme form 

 of Diplograptus, where the angle of thecal inclination has become 90°, 

 approaches to which are found in some other species of that genus and also 

 in some of Retiolites, as e. g. R , v e n o s u s from the American Clinton 

 beds. 



Retiograptus tentaculatus Hall 



Plate 16, figures 33-35 



Graptolithus tentaculatus Hall. Geol. Sur. Can. Rep't for 1857. p.l34 

 Eeteograptus te n tacula t us Hall. Geol. Sur. Can. decade 2. 1865. p.ll6f, 



pl.l4, iig.6-8 

 Retiograptus tentaculatus Matthew. Roy. Soc. Can. Proc. and Trans. 



1894. 11:114 

 Rctcograptus tentaculatus Gurley. Jour. Geol. 1896. 4:299 



