3 16 NEW YORK STATE MUSEUM 



The sicula is small and only visible in the obverse aspect. It is seen 

 in figures 239, 248 and 255. In 255 also the virgella and the mesial posi- 

 tion of the lateral spines on the first two thecae are distinctly shown. 

 Figures 241, 248, 253, 261 and others serve also to illustrate the mesial 

 position of the spines of the later thecae on the apex of the convex ventral 

 walls [see chapter on origin of spines, p. 69]. 



The four primary thecae have an alternate arrangement as in Dicello- 

 graptus, but the first and second thecae [see fig. 255, 259] grow only a 

 very short distance horizontally outward and bend then directly upward. 

 The later thecae grow in most species in two different series, which for the 

 remainder of the biserial section are coalescent and separated by a septum ; 

 the single " common canal " of the primary thecae becomes hence divided 

 into two. 



The septum is seen in several of the enlargements [see fig. 241]. 

 It is especially distinct in D. nicholsoni where it begins directly 

 above the sicula, while in D . s p i n i f e r [see fig. 265] the alternate budding 

 of the thecae appears to be continued much longer. Probably incorporated 

 in it is the nema which as figure 41 shows, was of considerable thickness 

 and by which the young rhabdosome was suspended until the time of the 

 separation of the coalescent stipes. It also served as support for the 

 ascending thecae of the biserial section, similarly as in the Diplograptidae. 



Emmons has in 1856 described two species as Cladograpsus 

 dissimilar is and in equal is. The former, from an mknown hori- 

 zon and locality, is figured [pi. 1, fig. 15] and based on a small fragment 

 of a Dicranograptus ; the latter, from Parrottsville, Term., is only described. 

 Both of these species should be dropped, the first, because its locality is 

 unknown and it can not be determined from the description and figure ; the 

 second, probably also a Dicranograptus, because the description is not 

 sufficient for determination. 



The British species of Dicranograptus have been arranged in the 

 Monograph of British Graptolites by the same principle as those of 

 Dicellograptus, i. e. by the form of the thecae. Only one of the lour groups 



