2l6 



HYDROZOA. 



and are isolated from one another, instead of overlapping. The 

 species of this genus are, like those of the preceding genera, re- 

 stricted to the Silurian rocks proper. 



The family Leptograptida has been founded by Lapworth for 

 a number of bilaterally-symmetrical branched Graptolites, which 

 agree with one another in having the hydrothecae attached along 



Fig. 98. — A, Rastrites peregrimis ; B, Rast rites capil- 

 laris ; c, Rastrites Liiincei; n, Fragment of Rastrites 

 peregrinns, greatly enlarged. All the figures are magni- 

 fied. Silurian. (Original.) 



Fig. 99. — Cocnograptus gracilis, 

 of the natural size, j Sicula. Ordo- 

 vician. (After Lapworth.) 



the whole of one face to the ccenosarc, and not overlapping. The 

 form of the polypary varies much in this family, consisting of two 

 long primary branches in Leptograptus itself, but being rendered 

 complex in Pleurograptus by the development of secondary branches 

 from each of the long primary stems, these in turn sometimes giving 

 off tertiary branches. In the beautiful genus Coenograptus (fig. 99), 

 again, the two primary stems originate from the centre of a tri- 

 angular sicula, and form an S- 

 shaped frond, the convex sides 

 of which give off simple branches. 

 All the above genera are found 

 in the Ordovician rocks, but do 

 not extend upwards into the 

 Silurian proper. 



The great family of the Dicho- 

 graptidce comprises a large number of branched Graptolites, all the 

 forms of which are found in the Upper Cambrian and Ordovician 

 deposits. The simplest type of this family is the genus Didymo- 

 graptus (figs. 92, D, and 100), in which the polypary consists of two 

 simple branches springing from a small pointed "sicula." The 

 angle included between the polypiferous sides of the two branches 

 ("angle of divergence") is always less than 180 , the opposite 



^^^ 



VVvVv^ 



Fig. 100. — Didyviografitics N-fractits. 

 Ordovician (Skiddaw Slates). 



