and Affinities of Graptolites. 



365 



externally, and having a solid slender rod (" virgula ") im- 



Fig. 1. 



Longitudinal section of a frag- 

 ment of Graptolites priodon, 

 after Barrande. 



bedded in the walls of the op- 

 posite side. ' This type form 

 (" monoprionidian ") is repre- 

 sented by the genus Graptolites 

 proper (fig. 1) , where the calicles 

 or tubular offsets from the com- 

 mon canal are in contact with 

 one another at their bases and 

 usually for a greater or less ex- 

 tent of their length, and by the 

 genus RastriteSj where they are 

 separated from one another by 

 considerable intervals. 



But we may conceive of two 

 such graptolites being united back to back ; and the resulting 

 form will then present two series of tubular offsets, one on 

 one side of the main tube and the other on the side dia- 

 metrically opposite, while the solid rod will now occupy the 

 axis, holding just such a position as it would do if it had 

 been formed by the union of the two rods of the component 

 halves. 



This form (" diprionidian ") is represented by such genera as 

 Dijylografptus^ where the tubular offsets stand out more or less 

 free from the sides of the main tube, and by ClimacograptuSj 

 where they are adnate to one another, so as to appear entirely 

 immersed in its walls. 



Some other forms also exist, such as DicranograjHuSj in 

 which the graptolite with a double row of denticles, after con- 

 tinuing its course for a time, divides into its component halves, 

 which then diverge from the basal portion as two branches, 

 constructed each on the single-rowed type. Branched single- 

 rowed forms ( Cladograptus^ Dicliograjptiis) also occur. In Di- 

 chograptus primary branches radiate from a common point at 

 the proximal end, where they are connected by a web-like 

 disk, apparently composed of a double membrane of the same 

 nature as that which forms the walls of the branches*. 



There are also some anomalous forms {Retiolites^ Phyllo- 

 graptus)^ whose structure has not yet been determined with 

 sufficient certainty to admit of a satisfactory association with 

 the true graptolites ; but the essential features in the moi-pho- 

 logy of the graptolites, as well as their more important modi- 

 fications, are expressed in the genera already cited. 



There is sufficient evidence to show that the graptolites 



See Hall, ' Graptolites of the Quebec Group.' 



