GRAPTOLITES OF NEW YORK, PART 2 117 



the sicula of Dicellograptus is of the same type as that of Leptograptus 

 and that the development of the proximal or sicular end of the rhabdosome 

 is essentially similar to that in Leptograptus as regards (i) the presence of 

 two crossing canals, and (2 ) the alternate development of the four primary 

 thecae. 



The spines which originally were apertural, because the aperture was 

 the most prominent part of the theca, wander backward in the Dicellograpti 

 owing to the bulging out of the mesial part of the theca through the sig- 

 moid curving of the latter, and again fix themselves on the most prominent 

 part of the theca, i. e. the mesial boss. The differentiation in the position 

 of the spines is for this reason also to be considered as lying in the line of 

 the general development of the Dicranograptidae. 



The whole rhabdosome of Leptograptus is typically more flexuous as 

 expressed in the name of its typical species L. flaccid us, and that of 

 Dicellograptus more rigid and in the whole, also much thicker. Owing to 

 the horizontal growth of the four primary thecae in Leptograptus and the 

 upward growth of the third and fourth theca in Dicellograptus, the former 

 has a wide open axil and the latter a narrower one, a differential character 

 which obviously leads from Leptograptus to Dicranograptus. 



If we remember that the Leptograptidae were probably derived from a 

 subdivision of Didymograptus and contrast the mode of fixation by nema 

 and primary disk in the latter with that which probably obtains among 

 the Dicranograptidae, viz, by the distal ends of the branches, it can be well 

 understood how the discarding of the original mode of fixation has brought 

 about all these changes. It leads to the origination of a strain in the 

 branches which has been overcome by their thickening, and to a still greater 

 strain in the sicular end of the rhabdosome which as we have noted before 

 [see p. 1 13] has led to the approach of the dorsal sides of the proximal parts 

 of the branches — or a diminution of the axil — and to the final coales- 

 cence of these sides in Dicranograptus, as well as to the adoption of a special 

 growth of the branches. 



The combination of the strong endeavor to hide and protect the aper- 



