GRAPTOLITOIDEA. 



219 



the manner in which the organism has been compressed, or the 

 particular view of the cell-apertures which may be afforded by a 

 given specimen (fig. 105, c, d, e). The species of the genus are 

 all Ordovician and Silurian. Of the remaining genera of the 

 Diplog7-aptidce the only one which needs special notice is the 

 remarkable Dimorphograptus (fig. 92, f), in which the proximal 

 portion of the polypary is monoprionidian, while the distal portion 

 is diprionidian. The polypary in its basal part thus resembles a 

 Monograptus, which genus it also approaches in the possession of 



Fig. 103. — A, Proximal portion 01 the polypary ot Dicranographis Nicholsoni, enlarged five 

 times ; b, Proximal portion of the polypary of Dicellograptus complanatus ; enlarged. Ordo- 

 vician. (After Hopkinson and Lapworth.) 



an adherent " sicula " ; while in its upper portion it has the char- 

 acters of an ordinary Diplograptus. The known species of Dimor- 

 -bhograptits are all Silurian. 



Of the remaining families of the Diprionidian Graptolites, the 

 Lasiograptidtz are not thoroughly understood, but their distinguish- 

 ing feature is found in the fact that the biserial polypary is provided 

 with peculiar chitinous spines, arising from the hydrothecae, which 

 subdivide and inosculate with one another so as to surround the 

 colony with a kind of netted fringe of fine fibres. The type-genus, 

 Lasiograptus, is Ordovician. The family of the Retiolitidce is also 

 a peculiar one, the characteristic feature being that the periderm is 

 very much attenuated, and is supported upon a remarkable network 



