I76 NEW YORK STATE MUSEUM 



branches [see fig. 82, 83]. These latter features are hence probably addi- 

 tional diagnostic characters of the genus. 



As diagnostic characters of the genus as far as they can be derived 

 from one species, we may then consider : the flabellate form, the presence 

 of processes on both sides of the branches and the union of these processes. 



The arrangement of the apertural spines on both sides of the branches 

 produces an aspect that is very suggestive of Acanthograptus and may 

 easily lead to the conclusion that Ptiograptus is an Acanthograptus whose 

 rhabdosome has become fixed by a close system of transverse dissepiments 

 and that this approach in aspect to Dictyonema is due to convergence. In 

 that case, however, the "spines" would have to be composed of bundles of 

 emptying thecae. This point could be cleared only by an investigation 

 of etched material — which with the single split rhabdosome in hand is 

 impossible— but it can be said that the spines have not the appearance of 

 composite bodies and that their uniting into new processes is also little 

 indicative of such composition. 



But granted that its relations were with Acanthograptus; in that case 

 it would be still generically differentiated by the uniting of the processes. 



Genotype : Ptiograptus percorrugatus sp. nov. 



Ptiograptus percorrugatus sp. nov. 



Plate 5, figure 6 



Description. Rhabdosome of medium size (67 mm high, 78 mm wide) 

 flabellate, branches quite uniformly .4 mm wide (.6 mm near base), very 

 closely arranged (6-7 in 10 mm, intervals .8 mm-i mm), subparallel, irregu- 

 larly curved, of extremely knotty and wrinkled appearance which is due 

 partly to the arrangement of the thecae which empty alternately on opposite 

 sides of the branches and give the latter thereby a zigzag direction ; and 

 partly to the presence of long apertural processes on the thecae. The latter 

 are bent obliquely forward or upward, apparently furcate at their extremities, 

 and overlap or unite frequently with those of the opposite side, the resulting 

 process growing forward midway between and subparallel to the branches 

 [see text fig. 82]. These processes together with the very thin, forward 



