FOSSIL CORALLINES. 47 



Mr. Carruthers published a " Revision " (now 

 obsolete, however), in which the leading types of 

 Graptolites are grouped as follows, beginning with 

 Rastrites : In this genus the polypary consists of a 

 simple, slender, hair-like tube, from which project a 

 series of detached cups (Jiydrothecce). (2) The old 

 genus, which was named by Linnaeus Graptolithus. 

 This has given the popular name to the entire group. 

 In it the polypary is simple, and the cups are so 

 thickly grouped along one side that they are all in 

 contact with one another (Fig. 31). (3) The genus 

 named Crytograptus by Mr. Carruthers, in which the 

 polypary grows in one direction, and gives off simple 

 or compound branches at intervals. (4) The genus 

 Didymograptus. In this we have, as it were, a twin 

 Graptolite, of a forked shape, with the cups arranged 

 within the fork (Fig. 31). The symmetrical forms 

 assumed by the coupled branches, or polyparies, are 

 various in different species. (5) Dichograptus ; a bila- 

 terally branched, and rebranched genus of Graptolites. 

 (6) Cladograptus ; another compound or bilaterally 

 branched genus. In this the branches often give rise 

 to other irregular branches, the first part of the name 

 signifying a branch. (7) Dendrograptus is a much- 

 branched, " tree-shaped," and rooted Sertularian-likc 

 Graptolite, with a thick main stem. It belongs to 

 the Cladophora. (8) Diplograptus (Fig. 33). In this 

 the cups are arranged on each side the axis, so as 

 to present the appearance of two single Graptolites 



