192 Mr. C. Lap worth on the Geological 



longest vertical range, and claims the most varied forms of the 

 Rhabdophora. It is the only division in which all the fami- 

 lies have biserial or diprionidian polyparies ; and to this section 

 alone the title of Diprionida can be properly applied. 



The third group (Didymograpta) rinds its type in the 

 genus Didymograptus, and includes all the genera at present 

 arranged in the two families of the Dichograptidee and Phyllo- 

 graptidas, according as their polypiferous branches are free or 

 conjoined. The form of the calycle and the mode of growth 

 of the polypary in all these genera is essentially similar ; and 

 they appear to be most naturally grouped in one and the same 

 primary division. The calycle is a conical sac expanding 

 outwards toward the aperture, which opens outwardly, well 

 outside the ventral margin of the polypary ; while in all the 

 typical genera the sicular angle is the wider or outer " angle 

 of divergence." Hence we may suggest for them the alter- 

 native title of the " Exoprionida." 



Finally we have a fourth section (Dicellograpta), which in- 

 cludes the families of the Dicranograptidas and Leptograptidse, 

 and of which the genus Dicellograptus is emphatically the type. 

 In all its component genera the calycle is free, narrow, and 

 flattened inwards upon the ccenosarcal canal, and the aperture 

 opens inwards either wholly (Dicranograpticlas) or in part 

 (Leptograptidas) within an excavation dug in the ventral 

 margin ; while the sicular angle is invariably the smaller or 

 inner angle of divergence. To this group, therefore, the 

 discretionary title of Endoprionida may be conveniently 

 applied. 



Turning next to our geological scale of the Lower Palaeo- 

 zoic rocks, we see that it is composed of the three grand rock- 

 systems of the Cambrian, Ordovician*, and Silurian, the boun- 

 daries of the middle system alone being defined with tolerable 

 exactness. The fossiliferous portion of the Cambrian, again, 

 is provisionally separable into a Lower or Paradoxidian divi- 

 sion, and an Upper or Olenidian division. The Ordovician 

 falls most naturally into two main divisions — a Lower or 

 Arenig division, and an Upper or Bala division — the line of 

 demarcation between them passing through the middle of the 

 so-called Llandeilo formation. Lastly, the Silurian is most 

 conveniently regarded as being composed of three members 

 — a Lower or Llandovery-Tarannon (Valentian) division, 

 a Middle or Wenlock-Lower-Ludlow (Salopian) division, 



* I have employed the title of Ordovician for the Lower Silurian of 

 Murchison throughout the whole of this paper for the sake of uniformity. 

 I prefer, however, the shorter and more euphonious title of Ordovian, 

 which I have generally employed elsewhere. (0. L.) 



