58 G NEW YORK STATE MUSEUM 



It must however be conceded that, as the initial thecae of the 

 branches (fig. 14) are similar to the stolonal thecae, a phylo- 

 genetic element, to be discussed later, enters into this problem. 

 But, even if to this latter element the principal weight in ex- 

 plaining the morphologic differences is given, the difference of 

 direction assumed by the stolonal and brachial thecae, and the 

 later thickening and functional change of the former, are suffi- 

 cient to indicate an important difference in the zooids that once 

 occupied the thecae. 



5 The stolonal thecae are more similar in shape and relative size 

 to the sicula of the colony than the brachial thecae. They widen in 

 a similar degree and possess the same simple apertural margin. 



In general, it may be said that all the thecae of a hydrosome 

 conform to some extent to the sicula of that hydrosome, forms 

 with long, slender sicula, having similar thecae and such with 

 wider, shorter sicula, as numerous Tetragrapti and Didy- 

 mograpti, having correspondingly shorter thecae; but at the 

 same time, the sicula of each colony is still relatively longer and 

 narrower than the average or extreme brachial theca. 



A comparison of the form of the thecae of the younger dicho- 

 graptid genera, as of Dichograptus, Tetragraptus and Didymo- 

 graptus, with that of the older and presumably also phylogene- 

 tically preceding genera, Bryograptus and Clonograptus, shows 

 that, in general, the older genera have the more tubular, simpler 

 thecae with less protracted apertural margins. It is, hence, 

 apparent that the stolonal thecae and the sicula represent the 

 older type of thecal form. 



6 The growth stages of the hydrosomes of Goniograptus 

 thureaui prove further that also tvithin each branch only 

 sicaloid thecae are at first produced (fig. 9, 10, 14). In fig. 14 

 the basal and distal parts of a branch of Goniograptus 

 have been still further enlarged to show their differences more 

 distinctly. The earlier thecae a, are tubular, lie subparallel 

 to the axis of the branch (the angle between the axis and their 

 outer margin is only 7°), overlap not more than one fourth of 

 their length ; have a straight aperture without marginal process, 



