HYDROZOA. 21 



the development of the polypary, which usually attaches the 

 polype to some foreign body. The organization of the soft 

 tissues is in general simple; the faculties of the Polypes are 

 very limited ; and the vital phenomena, save those of irrita- 

 bility and contractility, are inconspicuous. Nevertheless, the 

 influence of the combined powers of some of the species, in 

 adding to and modifying the crust of the earth, is neither 

 slight nor of limited extent. 



Class I.— HYDROZOA. :: 



Char. — Polypary, when present, flexible, external; for the 

 most part developing cells for the polypes according to 

 regular patterns. 



Family I. — GkaptolitidjE. 



To this class may probably belong the organic remains 

 called " Graptolites," which are exclusively and characteris- 

 tically Silurian fossils. A certain knowledge of their affinities 

 would require examination of the soft parts ; and the family 

 has long been extinct. Indications of the flexible consistency 

 of the polypary, and M. Barrande's statement of the existence 

 of a cylindrical canal in its axis, which he conjectures to have 

 contained the common connecting tissue of the polypes, have 

 weighed with the writer in placing the Graptolites provision- 

 ally in the present class of Polypi. The axis of the polypary is 

 sometimes straight (fig. 3, 3), sometimes spiral (fig. 3, 6). The 

 ordinary form, as given by the Graptolites priodon (fig. 3, 3), 

 is serrated on one side only, and is found abundantly in the 

 Cambrian or older Silurian beds of Scotland and Wales; it 

 occurs also in the Lucllow rocks. The double Graptolites 

 (Diplograpsus fig. 3, 5, and Didymograjmts fig. 3, 4) are Cam- 

 brian forms. Rastrites (fig. 3, 6) had the polypes only in one 

 * Op. cit. ed. 1843, p. 82. 



