GRAPTOUTES OF NEW YORK, PART 2 



71 



rimucronatus [see text fig. i]. In the genera Dicellograptus and 

 Dicranograptus the thecae possess a sigmoid curvature and the apertures 

 are situated in well defined excavations. The most exposed point of the 

 theca is hence the protruding buckle in the middle of the free portion of 

 the ventral margin [see text fig. 2], and there the spine is always found. 

 An analogous position of the spines is found in graptolites belonging 

 to an entirely different order Thus in excel- 

 lently preserved specimens of Climaco- 

 graptus p u t i 1 1 u s [see text fig. 3] it is seen 

 that at the point where the theca turns some- 

 what abruptly from an oblique direction to 

 one subparallel to the axis, a small (mesial) 

 mucro is developed and this is certainly the 

 most prominent place of the outer wall of the 

 theca. 



i Glosso. 

 graptus quadri- 

 mucronatus var. 

 cornutus nov. 

 r^. . .... M1 1 1 Ml ' Fragment of rhab- 



1 his principle is still more clearly lllus- dosome. x 5 



J 



trated by the distribution of the spines on the 

 rhabdosome. It is found that in most forms 

 the spines are restricted either to the mouth 

 of the sicula (virgella and other accompany- 



„, r \ Fi - e - 3 C 1 i m a - 



ing spines as in Cryptograptus, text figure 7) = <> s ^ & % f §. ut " 



, 1 r .1 1 1 mem of sicular end. 



or to the first two thecae, wherever by recurv- xs 



ing of the branches — in the Dichograptidae 



— or by the growth of the thecae in proximal Fig. 2 Dicranograptus nichoisoni 



var. parvangulus Gurley. Distal portion 



direction the sicular end is placed farthest of rhabdosome - *s 

 from the center of the colony. In genera like Bryograptus, Dichograptus, 

 Goniograptus, Tetragraptus, Didymograptus [see Mem. 7, pi. 5, 6, 7] where 

 the sicula comes to lie close to the primary disk, neither the sicula nor any 

 of the first thecae will as a rule develop any spines. Where, however, in 

 any species of these genera the branches of the rhabdosome make an abrupt 

 turn to attain the ascending direction discussed in Memoir 7, the most con- 

 vex part of the bent branch will develop spines. This is for instance shown 



