﻿50 BRITISH FOSSIL CRUSTACEA. 



" In ascending order the Old Red Sandstone, including all that portion of it which lies 

 above the conglomerate, and extends by Lesmahago to the Clyde and Lanark, is usually 

 of a lighter colour and freer quality than the subjacent beds, and occupies a very varied 

 outline in reference to the Carboniferous Limestone and Coal-fields on either side of it, 

 and under which it is seen to dip. On the banks of the Nethan Water, where we ex- 

 amined them, the junctions are much broken, and, on the whole, it would appear that the 

 older rocks have been so convoluted as to form the southern edge of the great central 

 Scottish Coal-field, or the complete girdle of the Douglas Coal-basin, the south-west side 

 of which is flanked by the Old Red of the Hawkshaw Hills, and also, according to Mr. 

 Slimon, by the Silurian rock of Bremerside Hill."* 



The following short account of the relation of the Pterygotus-hedx'mg shales of Les- 

 mahago was communicated in a letter to the Author by Mr. Slimon in 1867 : 



" Commencing in an ascending order with what we consider to be the lowest Silurian 

 beds of this area, we find them to be of great thickness, composed of coarse, hard, stony 

 bands, separated by thin clayey beds of shale. The Ceratiocaris is the principal fossil in 

 these lowest beds ; it is very abundant, but of small size, and is associated with some 

 shells. 



" Ceratiocaris recurs in the higher beds, but the specimens are far larger. A fault, 

 caused by the intrusion of a dyke of hornblende-porphyry, called the Nutberry Hill Dyke, 

 occurs here, and is followed by a rock containing an Orthoceras, but ill preserved. This 

 bed is succeeded by Ceratiocaris-skaies splitting up into slaty laminae, often as thin as 

 paper. Here the Ceratiocaris attains its maximum development, both in size and numbers, 

 becoming rarer in the higher beds, and disappearing altogether in the highest Silurian. 

 In these slaty beds Slimonia acuminata first appears, but very small in size ; as it increases 

 in development in the higher beds, Burypterus lanceolatus and Bterygotus bilobus (var. a) 

 make their appearance, both small in size and few in numbers in the lower beds, but 

 gradually increasing in importance in the higher ; var. a, inornatus, being replaced by 

 var. y, perornatus (attaining a length of from 2 to 3 feet). B. lanceolatus in the lowest 

 beds is not an inch in length, but in the highest it is of considerable size. 



"Burypterus scorpioides and Stylonurus Logani first appear in these upper slaty beds 

 (here much disturbed by intrusive hornblende-porphyry). They are more rarely met with 

 than other species, but appear to follow the same law as regards their gradual development 

 and decline. I have portions from higher beds attesting their continued increase 

 in size upwards. 



" The Upper Silurian beds here described occur along the Logan Water in a distance 



* The description of the Lower Carboniferous and Igneous Rocks of the tract is here omitted. 



