644 



NEW \OitK STATE MUSEUM 



Elles and Wood assert that in some forms, as in T . q u a d r i - 



^^^^ b V u c hiatus, the earliest thecae remain undivided, 



J^L \ /'jW ami the second or third theca on each side of the sicula 



^^mT ^^ undergoes division. The writer's material corroborates 



Y jT this statement, the correctness of which is already sug- 



srraius'" H*aa''sp.''"pro"i^ g«sted by the greater relative length of the bi'anches of 



mal pai-t o( a rhabdosome, ,1 n . i 



seen from the left side. x(i tlie lirst OrCler. 

 (Copy from Holm) 



We append here the description of a proposed sub- 

 genus which comprises group 7. 



ETAGRAPTUS' 

 Plate 9, figures 7-10 



The rhabdosome consists of two short central branches of the first 

 order, from which on either side originate two branches of the second 

 order, whicli, diverging in opposite directions and at right angles from 

 the former, appear as two slender, ilexuoua undivided branches, correspond- 

 ing to the vertical lines of an H, while the primary 

 brandies form the connecting bar. Tlie sicula is 

 long and the primary thecae diverge from the proxi- 

 mal part of the same. 



This group has the same structure as Tetra- 

 graptus, viz a twice repeated bifui-cation. It is, 

 however, in the character of its thecae, the point of 

 branching of the primary thecae and the direction 

 assumed by tlie branches of the second oi-der totally 

 different from the other species of Tetragraptus ; 



and it can be easily proved to belong to an entirely different series from the 

 other four branched forms [cli.lO, p.561]. The very slender thecae, the result- 

 ing very thin, ilexuous bi'anches, the peculiarly long sicula and, specially, the 

 divergence of the primary thecae at a point high up near the apex indi- 

 dicate that T. (Etagraptus) lentus, the type of the subgenus 

 belongs in one group with Goniograptus perflexilis, is closely 



Fig.oO Tetragrapt us similis 

 Hall sp. Proximal part seen from 

 below. Shows the apertures of the 

 sicula (in the center), of the first 

 two thecae and of those of the 

 proximal portions of the branches. 

 x6 (Copy from Holm) 



' In allusion to the similarity of its form to the Greek letter H. 



