440 THE SILURIAN ROCKS 



Orthoceras ibex, Sow. Ardmillan. 01. 



0. imbricatum, Wahl. Threave ; Penkill. C5-D3. 



0. ludense, Sow. Shalloch ; Threave. C3-C5. 



0. MacLareni, Salt. Straiten. Fl. 



0. mocktreense, Sow. Woodland Point. D2. 



0. multicinctum, Hall. Penkill. D3. 



0. pendens, Blake. Trochrague. B3. 



0. politum, M'Coy. Pinmore ; Penwhapple. B2-C2. 



0. primaevum, Forbes. Penkill. D3. 



0. subundulatum, Portl. Dow Hill ; Straiton. Cl-Fl. 



0. temiicinctum, Portl. Mulloch ; Woodland. D1-D2. 



0. tracheale, Sow. Woodland Point. D2. 



0. vagans, Salt. Shalloch Mill ; Threave. C3-C5. 



0. vaginatum, Schl. Ardwell. C2. 



Phragmoceras arcuatum, Sow. Mulloch Hill. Dl. 



Poterioceras sp. Mulloch ; Penkill. D1-D3. 



Trochoceras aspersum, Barr. (?). Woodland Point. D2. 



T. (Lituites) cornuarietes, Sow. Mulloch Hill. Dl. 



Trocholites planorbiformis, Com. Craighead. B2. 



LESMAHAGOW DISTRICT. 



To the north of the margin of the Silurian tableland and within the area 

 occupied by the Old Red Sandstone, there are several inliers of Upper 

 Silurian strata, ranging from Wenlock to Downtonian time, which have 

 been exposed at the surface by the folding and denudation of the younger 

 Palaeozoic rocks. These may be grouped as follows : 



1. The inlier near Lesmahagow (L in fossil list). 



2. The inlier of the Hagshaw Hills (H in fossil list). 



3. The inlier at Carmichael (C in fossil list). 



4. The inlier south of Tinto. 



Of the foregoing areas, by far the most important is that near Lesinaha- 

 gow, where the strata are arranged in a gentle anticline, only the northern 

 limb of which is preserved. The strata are admirably exposed in the Logan 

 Water, Birkenhead Burn, and adjoining streams. 



The passage-beds, comprising conglomerates and sandstones, with green 

 and red mudstones, overlying comformably the Ludlow rocks, and formerly 

 regarded as the base of the Old Red Sandstone, are now grouped with the 

 Silurian strata. They have yielded a fauna consisting of ostracods, phyllo- 

 carid crustaceans, eurypterids, and fishes ; some of the fishes being identical 

 with those in the underlying Ludlow rocks. The various subdivisions 

 adopted by the Geological Survey are given in the following table x : 



1 Memoir of the Geological Survey, "The Silurian Rocks of Scotland," p. 568. 



