220 



HYDR0Z0A. 



of chitinous fibres, thus producing a minutely latticed appearance 

 of the surface (fig. 106). According to Lapworth, the two virgulse 

 of the biserial polypary are separated, and are attached to opposite 

 sides of the peridermal network. The type-genus is Retiolites itself, 

 the species of which are Ordovician and Silurian. Lastly, the family 

 of the Phyllograptida includes the single genus Phyllograptus, all 

 the species of which are confined to the lower portion of the 

 Ordovician system. In this genus (fig. 107) the polypary is leaf- 



Fig. 104. — A, Diplograptus folia- 

 ceus, slightly enlarged ; B, A Diplo- 

 graptus showing two long lateral 

 spines, in addition to a central 

 "radicle"; c, Base of a Diplo- 

 graptus, enlarged. Ordovician. 

 (Original.) 



Fig. 105. — A, A specimen otClimacograptus normalis, 

 in relief and partially split in half, showing the virgula 

 embedded between the two branches of the duplex 

 polypary ; b, Transverse section of the polypary of the 

 same ; c, d, and e, Different views of the polypary of 

 the same, showing the appearances produced by differ- 

 ences in the direction of compression. r, Radicle ; 

 v, Virgula; /, Septum between the two halves of the 

 polypary; k, Hydrotheca ; m, Aperture. All the fig- 

 ures are enlarged. Silurian. (Original.) 



like, and consists of four rows of hydrothecae placed back to back, 

 in such a manner as to resemble two Diplograpti intersecting one 

 another at right angles. In consequence of this peculiar structure 

 of the polypary, the Phyllograpti are sometimes spoken of as the 

 " tetraprionidian " Graptolites. 



The Graptolites have been considered with some detail on account 

 of their great importance in determining the chronological succession 



