0RT1IIS. 131 



that the larger impressions, termed divaricator (r), in our figures of this species (as well 

 as of 0. Michclini), is apparently composed of two parts, the anterior or central being the 

 cardinal or divaricator, while the other, the posterior or lateral (which is parallel), may 

 belong to the ventral adjuster? 



In the dorsal valve, the fissure is almost entirely occupied by a moderately produced 

 shelly prominence or cardinal process, to which were no doubt affixed the divaricator 

 muscular fibres. The inner socket walls are somewhat prolonged under the shape of pro- 

 jecting laminae, and to the extremity of which free spiral arms may perhaps have been 

 attached (?); while under this shelly process a longitudinal ridge, with a wide, flattened space 

 on either side, separates the quadruple impressions of the adductor or occlusor muscles ; 

 these last producing two oval-shaped depressions, placed obliquely one above the other, 

 and separated by lateral elevations branching from the central ridge. Vascular impressions 

 and ovarian markings are at times clearly observable in the interior of both valves. 

 Dimensions and degree of convexity very variable two extremes have measured — 

 Length, 32; width, 41 ; depth, 21 lines (the largest example I have seen). 

 „ 16; „ 17| » 16 „ 

 Obs. 0. resupinata is a common, well-known, and widely spread Carboniferous shell. Its 

 exterior appearance has been often described and illustrated, but not always properly under- 

 stood. In shape it is subject to considerable variation, and hence the specific denominations 

 of resupinata, connivens, gibbera, and latissima, which appear to Mr. Morris and myself to 

 have been applied to what we must regard as different states of a single species ; and in my 

 plates will be found represented some of the most remarkable shapes. The striation is also 

 finer in some specimens than in others ; and certain examples show, more than others do, the 

 small drop-like elevations from which originate the little thread-like spines, which are 

 generally broken close to the surface of the valves in a large number of specimens. 



Interiorly, some unimportant variation in detail may likewise be occasionally observed 

 and which I have endeavoured to illustrate. 



In England, 0. resupinata abounds in the Carboniferous limestone and shales of 

 many localities, such as at Greenhow Hill ; Hawes, Otterburn ; at Dovedale, in Derbyshire ; 

 the Isle of Man ; Lowick, Northumberland ; Settle, Malham Moor, Withgell, Yorkshire ; 

 at Ulverston, Bolland, &c. 



In Ireland, it is found at Tyrone, St. John's Point, Dunkineely, Cornacarrow, Ennis- 

 killen ; Cruiceroth, near Drogheda, County Meath; Little Island; Cornagrade, Bundoran, 

 Cookstown, Millecent, &c. 



In Scotland, it occurs at Gare, Raes Gill, Middleholm and Brockley, near Lesmahago, 

 Capel Rig, East Kilbride, &c, in Lanarkshire ; in Ayrshire, at Auchenskeigh, near Dairy, 

 West Broadstone, Beith, &c ; in Dumbartonshire, at Castlecary ; in Stirlingshire, at 

 Balglass Burn, as well as in the Campsie Main Limestone and Corrieburn beds. It 

 occurs also in Midlothian and other Scottish counties. 



On the Continent, it is not rare at Vise and Toumay, in Belgium, &c. 



