GRAPTOLITES OF NEW YORK, PART 2 I23 



lures. The connecting species of Trenton age has, however, not yet 

 been observed, the development of the Utica species Gl ossograptus 

 eucharis having obviously taken place in the Atlantic basin. Gl osso- 

 grap t u s c i 1 i a t u s, the genotype and representative of the genus in the 

 Normanskill shale does not — as noted before — suggest by its form any 

 relationship to G . qua d rimucronat u s, but appears rather as a direct 

 descendant of G. e chin at us, a Deepkill form, which it resembles in all 

 important features. 



A form which in its external appearance by its subrectangular section, 

 smooth broad lateral sides and the form of the aperture \sce pi. 31, fig. 17J, 

 as well as in its peridermal skeleton [ibid. fig. 16] also recalls G. q u ad ri- 

 mucronat u s is Clathrograptus g e i n i t z i a n u s Hall, one of our 

 most interesting Normanskill forms, but also one of the most difficult of 

 reconstruction. It possesses a strong outer periderm, a system of ledges 

 corresponding to that of G . qua d r imucronatu s but apparently lacks 

 the network of fibers, the apertural spines and the virgula, and since it also 

 appears before G. quadrirrtucronatus quite probably represents an 

 independent but subparallel development wholly induced by the tendency 

 to a lightening of the periderm. While in Glossograptus the virgula is 

 still well preserved besides the two lateral axes \_sce pi. 26, fig. 9], in Clath- 

 rograptus geinitzianus only two zigzag shaped lateral axes have 

 been observed. 



The Siluric genus Retiolites represented here by our Clinton species 

 R. v e n o s u s possesses a like framework of ledges as Clathrograptus with 

 the distinction that one of the lateral ones is zigzag shaped and the other 

 straight [pi. 31, fig. 8j. It lacks all spines and the reduction of the con- 

 tinuous periderm to a fibrous network covered by a very thin cuticle is 

 distinctly intended to reduce the weight of the periderm. Both genera, 

 Retiolites which has the thecae of a Diplograptus, and Gothograptus which 

 has those of a Climacograptus have apparently independently developed 

 from the genera mentioned here with them, and after them, viz, in Siluric 

 time. 



