294 PALAEONTOLOGY 



a float, and that numerous siculae were developed in 

 reproductive vesicles under the float and grew out from 

 these as a second generation of polyparies (Fig. 88, g). 



Families GLOSSOGRAPTID^E and RETIOLITIDS are 

 allied to the last, but show peculiar developments of the 

 test firstly, in a change from a membrane of uniform 

 thickness to one with strengthening ribs, at last becoming 

 a complex network, in the meshes of which is left a 

 membrane so delicate that it is lost in the fossil state ; 

 secondly, in a great development of projecting spines. 

 The latter form the special feature of the Glossograptidce 

 (extending through nearly the whole Ordovician), the 

 network being most highly developed in Retiolitidce 

 (almost confined to Silurian). 



Family DIMORPHOGRAPTID.E is a small group generally 

 spoken of as transitional between Diplograptidce and 

 Monograptidce. They may either be regarded as mono- 

 graptids which revert to a diplograptid stage, or as 

 diplograptids in which the formation of the second 

 column is delayed, as they are uniserial only in the early 

 part of the polypary and biserial afterwards. The only 

 genus, Dimorphograptus, is confined to the Lower Valentian, 

 being contemporaneous with the earliest monograptids. 



Family MONOGRAPTIDS. Uniserial and unilateral. 

 The characteristic Silurian family, ranging from bottom 

 to top of that system. 



The chief genus, Monogvaptus (Figs. 87 ; 88, h-m), is 

 divided by Miss Eiles and Miss Wood into the following 

 seven groups, characterized by differences in the thecae, 

 and named after a type-species : 



1. M. cyphus : thecae simple, straight, overlapping 

 tubes, with even apertural margins (Middle Valentian 

 to top Salopian ; Fig. 88, A.) 



2. M. sandersoni : thecae with flowing sigmoid curvature 

 and oblique apertural margins (Lower and Middle 

 Valentian ; Fig. 88, *'). . 



