714 



ME. J. E. ITAEE AN"!) DE. H. A. NICHOLSON 



a genus which is common in Ordovician rocks. Two species are, 

 however, found in the Silurian, viz. : — Ampyx parvulus^ Porhes, from 

 the Lower Ludlow rocks of Ludlow, and Ampyx Roualti, Barr., from 

 the corresponding beds of Bohemia. It is interesting to find another 

 form which reduces the gap between the newest Ampyx of the Ordovi- 

 cian rocks and these diminutive forms of the Ludlow series. 



The fauna that most nearly approaches our Trilobite fauna of the 

 Stockdale Shales is found in the Tarannon Shales of the Onnj- Eiver. 

 Animiber of Trilobites from these beds are preserved in the Museum 

 of Practical Geology, and we have examined the specimens ; they 

 are : — 



Pbacops glaber, n. var. 

 Cheirurus bimucroBatus, Murch. 

 Enerinurus punctatus, var. are- 

 naceus. 



Calymene BlumenbacMi, Brongn. 

 Illagnus Tliompsoni, var. 

 Proetus nasiger, Edgell, MSS. 

 Acidaspis dama, Fl. ^~ Salt.? 



The first form' occurs also in the Stockdale Shales ; of the two 

 latter, Proetus nasiger is very near our P. hracliypygus. It appears to 

 possess a narrower tail, and larger basal lobes to the glabella. The 

 specimen doubtfully referred to Acidaspis daraa is near to our A. 

 erinaceus. The glabella and free cheek onl}- are seen in the specimens 

 of the lluseum of Practical Geology. The former is smooth, and the 

 lobes are slightly difi'erent from our forms. The discovery of more 

 specimens may prove that these distinctions are merely varietal in 

 the case both of the Proetus and of the Acidaspis. 



VI. E,E]iIAEKS ox THE BeAEINGS OF THE ReSTJLTS. 



The Stockdale Shales have been shown to consist of from two- 

 hundred and fifty to four hundred feet of alternating black and green 

 shales, blue mudstones, often calcareous, and greenish-grey grits. 



They are divisible into a Lower group, the Skelgill Beds, consisting 

 mainly of dark Graptolite-bearing shales alternating with lighter 

 mudstones, which are entirely devoid of Graptohtes except where 

 they pass into the adjacent Graptolitic shales, and an Upper 

 group having from twice to thi'ee times the thickness of the 

 Lower one (but probably formed much more rapidly, and therefore 

 not of anything like the actual importance of the Lower group), con- 

 sisting chiefly of green and purple shales with interstratified grit- 

 bands, and a few insignificant seams of dark Graptolite-bearing 

 shales. 



The Stockdale Shales are furthermore capable of being divided 

 into a series of zones, recognizable by their lithological characters 

 and also by their contained fossils, these zones undergoing only a 

 slight alteration in thickness and character when traced across the 

 country. 



The lowest zone has been shown to be entirely conformable to the 

 Ashgill Shales below, there being no discordance of strike, and the 

 same bed of the Ashgill Shales being seen, in several remote sections, 

 with the lowest band of the Stockdale Shales resting directly upon it. 



