644 



NEW lOnK STATE MUSEUM 



Elles and Wood assert that in some forms, as in T. quadri- 

 L r a c h i a, t ii s , the earliest thecae remain undivided, 

 and the second or third theca on each side of the sicula 

 undergoes division. The writer's material corroborates 

 this statement, the correctness of which is already sug- 

 gested Ly the greater relative length of the branches of 

 the first order. 



We append here the description of a proposed sub- 



F)g'.49 T e t r a 8 T a p t \i s 

 si mi lis Hall sp. Proxi- 

 mal part of a rhabdosome, 

 seen from the left side, xti 

 (Copy from Holm) 



genus which comprises group 7. 



ETAGRAPTUSi 



Plate 9, figures 7-10 



The rhabdosome consists of two short central branches of the first 

 order, from \vhich 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, liexnous undivided branches, correspond- 

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

 branches form the connecting bar. The sicula is 

 long and the })rimary thecae diverge from the proxi- 

 mal part of the same. 



This group has the same structure as Tetra- 

 graptus, viz a twice repeated bifurcation. It is, 

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

 branching of the primary thecae and the direction 



two thecae and of those of the 



assumed by tlie branches of the second order totally $6°f(^™fj! i^rom n'oimV''^ branches. 

 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 [ch.lO, p.561]. The very slender thecae, the result- 

 ing very thin, llexuous branches, the peculiarly long sicula and, specially, the 

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

 dicate that T . ( E t a g r a p t u s ) 1 e n t u s , the type of the subgenus 

 belongs in one group with Goniograptus perflexilis, is closely 



T"ig-..50 Tetragraptus similis 

 Hall sp. Proximal part seen from 

 below. Shows the apertures of the 

 sicula (in the center), of the first 



'In allubion to the similarity of its form to the Greek letter PI. 



