336 NEW YORK STATE MUSEUM 



fragmentary specimens, retaining about one half of the first loop. Curi- 

 ously enough this is by far the most common mode of occurrence of 

 this species and whole slabs may bear nothing but these fragments. Elles 

 and Wood's statement that " the stipes of var. mini m u s have never been 

 observed to meet, whereas in D. furcatus they typically cross," indi- 

 cates that also in the British variety the fragmentary occurrence is the more 

 common one. Specimens of D. furcatus which cross several times 

 have been seen by me only in the Stockport collection [see fig. 268, 269]. 

 Gurley (1896) has investigated these and concluded fro~i the alternate 

 position of the thecae, in the compressed state, on the inside and outside of 

 the loops, from the crossing of the branches at different levels and their 

 crossing alternately over and under, that they " originally grew spirally 

 upward, each describing an oppositely directed curve," adding that judging 

 from the figures, probably also Lapworth's species Dicellogr. cad li- 

 ce us and Dicranogr. ziczac grew in continuous spirals. We have 

 at another place [p. 112] brought forward evidence to prove that the spiral 

 growth of the branches was a general feature in the Dicranograptidae. 



The periderm of the rhabdosome must have been very thick in this 

 species, if we judge rightly from the strongly glossy appearance and the 

 smaller amount of compression, as compared with the associate species, 

 features which are especially apparent in the Glenmont suite of specimens. 

 That this thickness of the test was in some way connected causally with the 

 relative compactness of the spirals in D. furcatus, is hardly to be 

 doubted. 



In D. furcatus the convexity of the distal part of the theca and 

 the introversion and introtorsion of the apertural part appear to have 

 reached their extreme development. On account of these short, high 

 bosses of the thecae and the thickness of the periderm the branches have 

 in their ventral and dorsal aspects a curious beaded or shortly segmented 

 appearance which is quite characteristic of the species [see right branch 

 on fig. 270]. 



