IIOLOGRAPTUS. 75 



Tlicnv narrow tubes of tlic Dicliograptid typ(\ 



The t'oi'in of tlie polypary in Jfoloijraptas is closely related to that of 

 Troi'Jiograptiis, differing-, so far as is known, only in the fact that the four main 

 stipes (stipes of the second order) apparently throw off lateral l^ranches from l)oth 

 sides. These lateral branclies may be either simple or compound. 



While at first sight there seems to be no doubt that the lateral l)ranches are 

 given off from both sides of the four main stipes, yet it is impossible to prove that 

 this was the original mode of growth. An examination of the left-hand branch 

 drawn on fig. 2 a, PI. VIII, seems to lend some support to the suggestion that the 

 lateral branches may have really originated from one side only (thecal margin). 

 If this should prove to be invariably the case, then TrofJiorjraptm and llohxjrajjlns 

 could no longer be regarded as separate genera. Whether this be so or not, there 

 seems to be no doubt that the genus llouvilligraptus of Barrois is identical with the 

 Hologmptus of Holm. 



The Holograpti attain an extraordinary size, exceeding that of any other 

 Graptolite genus with the exception of Tcnmograptus. Our British examples 

 must have been, when fully grown, more than 80 cm. in length ; and the branches, if 

 disposed regularly and spread out symmetrically, must have extended over a circular 

 area of that diameter. There appears to be no difference in the width of the 

 stipes of the first and second order, or of the lateral branches. The distance 

 between the points of origin of the lateral branches on the four main stipes is 

 large near the sicula, and appears to decrease irregularly towards the distal 

 extremity, where the lateral branches are approximate and numerous. A few of 

 these lateral branches show a tendency to re-branch in their turn, but none are 

 known to possess more than two terminal branches. 



The form of the thecce is, as a rule, obscure. 



Holograptus Deani, Lapworth, MS. Plate VIII, figs. 2 a — c. 



Main stipes and compound lateral branches somewhat curved ; main stipes 

 about 40 cm. in length, robust and having an average width of 2 mm. 

 Lateral branches arising from both sides of the four main stipes and 

 curved in the same direction, usually at an angle of about 60°. Distance 

 between the lateral branches decreasing with remoteness from sicula, as 

 does also the angle of divergence. Compound lateral branclies short, rare. 

 Theca3 of the ordinary Dichograptid type, nine in 10 mm., angle of inclina- 

 tion 35° — 45°, average length about 2*5 mm. 

 Description. — The distance between the points of origin of the lateral branches 

 on the main stipes decreases, as a rule, from an average of about 6 cm. near the 

 sicula to about 1 cm. near the distal ends. A larger proportion of lateral branches 



