466 NEW YORK STATE MUSEUM 



the junctions of the parietal and mouth ledges. The thecal walls which 

 within the meshes do not seem to have consisted of any hard parts at all 

 would, according' to their apertural parts seen in this specimen, appear to 

 have had the position of those of a Diplograptus. The apertures in this 

 specimen and in plate 31, figure 10 do not occupy the full size of the aper- 

 tural meshes of the skeleton but appear as rather low transversal openings. 



Plate 31, figure 13 illustrates well the gradual thinning out of the con- 

 tinuous periderm in antisicular direction. This observation and the one 

 that young individuals show the skeleton only, lead to the inference that 

 the continuous peridermal layer was a gerontic feature of the individual 

 thecae, for even those which have reached full maturity in size (as in 13) 

 do not yet possess it. The nemacaulus has so far not been observed either 

 within the rhabdosome or as antisicular prolongation. It may have been 

 incorporated in one of the six longitudinal structure lines of the rhabdo- 

 some, possibly in one of the zigzag axes in the middle of the lateral 

 faces. 



Plate 31, figures 16 and 17 represent reconstructions of this form, 

 without and with continuous periderm respectively. 



As regards the systematic position of this form, we do not dispute 

 Hall's and Gurley's view that it is congeneric with R e t i o g r a p t u s t e n- 

 t a c u 1 a t u s, for the reason that the structure of the latter is not sufficiently 

 known to permit a definite diagnosis of the genus, and the conclusion of 

 the congeneric relation of the two species is based mainly on their exterior 

 similarity.' 



The structure of this Retiograptus agrees with that of the majority of the 

 Rettolitidae in the general structure lines but differs from that of the typical, 

 later genera as Retiolites, Stomatograptus and Gothograptus in the greater 

 regularity of the arrangement of the ledges, the absence of the smaller 

 meshes and the presence of a continuous periderm (or rather greater thick- 

 ness of the latter). It appears in all these features as a more primitive 

 retiolitid and is obviously more closely related to Glossograptus, and 

 especially to such forms as G . q u a d r i m u c r o n a t u s which it resembles 



