GRAPTOLITES OF NEW YORK, PART 2 75 



its species and the latter by the size and the great number of their indi- 

 viduals sufficient proof of undiminished vital power. The spine growth 

 begins already vigorously with the sicula, which has at least four spines 

 [see text fig. 8], two of which are continued in the series on the lateral faces, 

 the others in the apertural spines. For these reasons I believe that we have 

 here an unmistakable instance of the factor which Beecher has denoted as 

 production of spines by repetition. In regard to this agency he says : 



When from any cause, the forces of nutrition are directed toward spine 

 production, and when the direct results are accomplished in the reciprocal 

 formation of one or more spines, there is often an apparent inductive influ- 

 ence or impulse given to growth toward the further production or repetition 

 of spines. This may result in the formation of compound spines, or a group 

 of spines, or even produce a generally spinous condition. 



There is little doubt that the first impulse in the case of Glossograptus 

 was given by external stimuli. These resulted in the 

 formation of spines around the apertures of the thecae, 

 a condition that was carried back to the sicula and 

 thence spread over the lateral sides of the rhabdo- 

 somes. 1 The same is true, we surmise, in the cases of 

 Tetragraptus acanthonotus and D i d y m o - 

 graptus spinosus but since the sicula of neither 

 ot the two has been observed, there is no direct graptus ciiiatus 



' Emmons. Young 



evidence bearing on this problem. 



An interesting case of inceptive or intermittent 

 repetition is that observed in the variety of Glosso- 

 graptus quadrimucronatus occurring accord- Fig. 9 Glossograptus 



quadrimucronatus (Hall). 



ing to Lapworth in the Irish graptolite shales, where Copy from La P worth - Nat. size 

 large spurs like those of the aperture of the sicula appear again upon the 

 lateral sides opposite the fourth or fifth theca [see text fig. 9]. The points 

 of their appearance are not nearly as exposed as the sicular extremity of the 

 rhabdosome or the apertures of the thecae, and not more than any other 



1 It is to be noted in this connection that the forms which have apertural spines only, 

 possess only one or two sicular spines, as Dipl. foliaceus. 



