GRAPTOLITES OF NEW YORK. PART 2 III 



The approach of certain Dicellograpti to Dicranograptus manifests 



itself in still another feature, that is quite apparent in Di cello g r. sex- 

 tans, anceps and mo ft at en sis (cf. I), moffatensis var. 

 alabamensis described here), namely in the obliquely outward and 

 upward growth of the four first thecae, whereby a structure exactly cor- 

 responding to that in'the proximal part of Dicranograptus is produced. D. 

 sextans has probably for this reason been at one time referred to Dicran- 

 ograptus [Hopkinson 1870], but as Elles and Wood [1904, p. 154] have 

 pointed out in the description of that species: "This form of the proximal 

 end, is practically identical with that of some other species of Dicellograptus, 

 as for example, D . anceps, and varies so little from the more ordinary 

 type exemplified in D. divaricatus, etc., that there is no reason to 

 exclude D. sextans from the genus Dicellograptus on this account," 

 adding, " it ma) r , however, be regarded as one of the intermediate forms 

 connecting this genus with that of Dicranograptus." 



Dicellograptus. smith i also possesses this " diplograptid " 

 structure of the sicular end in a marked degree. Its combination there 

 with the subparallel direction of the proximal parts of the branches gains a 

 special significance in regard to the phylogeny of Dicranograptus, if it is 

 remembered that also in the biserial portion of most species of Dicrano- 

 graptus but the four first thecae have an alternate arrangement, while the 

 following are growing in two uniserial series and are separated by a septum. 

 The adnascence of the dorsal walls of D . s m i t h i , already so much 

 approached is, hence, all that is needed to produce a perfect Dicranograptus 

 structure. It is certainly also quite significant that just those species men- 

 tioned above as possessing the " diplograptid " structure of the four proximal 

 thecae are also exactly the forms that most distinctly exhibit the tendency 

 to a parallelism or concave curvature, i. e. to a mutual approaching of 

 the proximal parts of the branches. It is, therefore, proper to infer that 

 these species, D . sextans, D . s m i t h i , D . anceps and D . mof- 

 fatensis are all stages in the evolution of Dicranograpti. But since D. 

 anceps belongs to an entirely different group from the others — having 



