Mr. C. Lapworth on new British Graptolites. 163 



18. Dicellograptus divaricatus, Hall, var. rigidus, Lapw. 

 (PI. V. figs. 20a, 20 b.) 



Dicellograptus myffatensis, var. divaricatus, Hall, Lapworth, Grapto- 

 lites County Down, Proceedings Belfast Nat. Field-Club, 1876, 

 pi. vii. fig. 10, &c. 



Branches of polypary stout and rigid, divaricating at a 

 medium angle. Axil narrow and deep, bluntly pointed proxi- 

 mally, frequently extended distally by a chitinous film uniting 

 the dorsal surface of the earlier portions of the branches. 

 Axillary spine, radicle, and lateral spurs rarely visible. Hy- 

 drothecae twenty-four to the inch, short, free for about one 

 third of their length ; free portion of each so strongly intro- 

 verted that the ventral margin of the branch appears un- 

 broken. 



The branches in typical examples of this form are tolerably 

 straight, and are of an equal width of about one sixteenth of 

 an inch throughout. They diverge at an average angle of 

 about 60°. In a few examples there is a well-marked out- 

 ward curve within a short distance of the axil j and this is 

 occasionally succeeded by a general inward curvature of long 

 radius. 



As a rule the axillary portion of the polypary forms a small 

 triangle. There is frequently a short radicle ; but only in 

 very young forms is there a trace of either axillary spine or 

 lateral spurs. A thick web of chitinous matter unites the 

 dorsal edges of the two branches as far as a point opposite 

 the aperture of the third hydrotheca. 



The vast majority of specimens afford no trace of the 

 presence of hydrothecas beyond a slight crenulation of the 

 ventral margin of the branch, both ventral and dorsal edges 

 appearing equally smooth and unbroken. This may be called 

 the characteristic aspect of the species. In the opposite or 

 obverse aspect the form and position of the thecas can often be 

 made out with certainty. They agree very closely with those 

 in Dicellograptus intortus, the free portion of each being strongly 

 introverted, while the apertural margin is inclined, and opens 

 wholly within the ventral margin. It is pressed so closely 

 against the base of the succeeding theca that the excavation 

 is only discernible as a faint slit running obliquely across the 

 body of the polypary. 



This is the form I have generally referred to Hall's Grapto- 

 lithus divaricatus (Grapt. Quebec Group, p. 14, fig. 14, upper 

 figure) . It is the only British form known to myself that can be 

 even with doubt referred to his species. If his figure is correctly 

 drawn, however, the British and American forms must be dis- 



