GRAPTOLITOIDEA. 



215 



two separate coenosarcal tubes which are closely apposed to one 

 another, back to back, along their entire length. 



Before briefly considering some of the leading types of these 

 two great groups, attention may be directed 

 for a moment to the singular genus Cory- 

 iwides (fig. 96), which differs in important III//, 



respects from all the ordinary Graptolites. 

 In this curious form the organism consists 

 of a cylindrical chitinous tube, tapering to- 

 wards the base, where it is furnished with 

 two small spines, and expanding above into 

 a species of toothed cup. The polypary 



does not show 



any 



of having been 



Fig. 96. — Corynoides cali- 

 cularis, enlarged. (Origi- 

 nal.) 



attached to any foreign body, and the sug- 

 gestion has been thrown out by Lapworth, 

 that the type may be regarded as a 



Graptolite which never proceeded beyond the stage of a " sicula." 

 The type of the Monoprionidian Graptolites is the genus Mono- 

 graptus itself (figs. 93 and 94), which is also the central genus of the 

 family of the Monograptidcs. In this genus the polypary is simple and 

 linear, possessing but a single row of hydrothecas on one side, and 

 commencing by an attenuated, often curved base. The polypary is 

 straight, curved, or helicoid in different forms ; and great variations 

 exist in the shape of the hydrothecse in different species (fig. 94), 

 these structures usually, but not always, overlapping to a greater or 

 less extent. The " sicula " 

 is attached along the dor- 

 sal margin of the proximal 

 end of the polypary (fig. 

 92, b). All the species of 

 Monograptus are confined 

 to the Silurian rocks proper 

 (the Upper Silurian of Mur- 

 chison), ranging from the 

 base to the summit of the 

 system. In the allied genus 

 Cyrtograptus (fig. 97) the 

 polypary is also unilateral, 

 in so far as the hydrothecse 

 are produced from the 

 sicula in one direction only, 

 but the polypary is now 

 branched. This genus is also confined to the Silurian rocks proper. 

 Again, in the genus Rastrites (fig. 98) the polypary resembles that of 

 Monograptus in form, but the hydrothecse are more or less linear, 





Fig- 97- 



-Cyrtograptus MtircJriso7ii, Carr. 

 (After Carruthers.) 



