640 J. HOPKINSON AND C. LAPWOKTH ON TB'E GEAPTOLITES OP 



The two main branches of each bilateral Graptoloid originate from 

 approximately opposite points, at, or near, the broad end of the sicula, 

 and either grow backward along its length, almost completely 

 imbedding it, and coalesce by their dorsal walls into a diprionidian 

 or tetraprionidian structure, or, proceeding freely outwards at va- 

 rious angles, give rise to a symmetrical, simple or compound mono- 

 prionidian polypary. The sicula meanwhile, when not imbedded, 

 either gives origin to a central branch, is absorbed, or, increasing 

 somewhat in size, retains permanently the precise form and pro- 

 portions it possessed at the period when the main branches were given 

 off. This persistent sicula forms a very conspicuous feature in several 

 monoprionidian genera, and constitutes the " axillary spine " of some 

 authors, and the " radicle " (in part) of others. 



In all the siculate Graptolites the mode of origin of the branches 

 is almost identical. The notable differences observable in the ulti- 

 mate form assumed by the polypary in the various genera, are all 

 primarily due merely to the direction assumed by the main branches 

 as they grow onward from their point of origin in the sicula. This 

 direction remains much the same when our observations are re- 

 stricted to a single genus ; but within the limits of the section of the 

 Graptoloidea every gradation is recognizable, from forms in which 

 the dorsal walls of the two primary branches are wholly coalescent, 

 to forms in which the ventral margins are almost in contact. The 

 actual relations of the sicula and branches, however, remain invari- 

 ably the same throughout. The major extremity of the sicula being 

 always situated on the same side of the axil as the ventral margins 

 of the branches, its minor extremity consequently invariably bisects 

 the angle formed by their dorsal margins. The fact that the major 

 extremity of the sicula forms the proximal end of the complete poly- 

 pary in Dijplograptus and Dicranograptus decides conclusively the 

 controverted question as to whether the dorsal or the ventral margin 

 in Dicellograptus and Didymograptus ought properly to be regarded 

 as proximal. In future, therefore, the whole of the Graptoloidea, 

 without exception, must be figured with the major extremity of the 

 sicula placed below — or, in other words, with the pointed end of the 

 sicula directed upwards. 



This conclusion also determines that the dorsal angle is invariably 

 the theoretical "angle of divergence;" for in Dvplograptus and Clima- 

 cograptus it is unquestionably 0°, the polypary in these genera being 

 composed, actually or theoretically, of two branches which do not 

 diverge at all from each other, but are conjoined for the whole of 

 their length. In Dicranograptus the branches are conjoined for some 

 distance and then diverge at an angle of less than 180°. In the 

 closely allied genus Dicellograptus the branches are non-coalescent, 

 and diverge at an angle of less than 180° immediately from their 

 point of origin. Through the genus Leptograptus and its allies, in 

 which the angle of divergence always approximates to 180°, we pass 

 from Dicellograptus to the homomorphous but very distinct genus Di- 

 dymograptus, in which the angle of divergence is gradually carried 

 through the various species from 180° almost to 360°, the branches 



