100 



THE GEOLOGIST. 



strife, wliicli increase in number b^^ intercalations, or bifurcations, at variable 

 distances from the beaks ; and at intervals the striae themselves increase in 

 thickness and prominence, giving birth to smaU hollow spinose asperities, or 

 thread-like tubular spines, which augment in number towards the margin, but 

 are broken close to the surface of the valves in the generality of specimens. 

 The intimate sheU-structure is also perforated by innumerable canals, of which 

 the exterior orifices, in the shape of minute punctures, cover the entire surface 

 of the shell. 



In the interior of the ventral valve, the dental plates extend to some distance 

 along the bottom of the sheU, and between these a smaU rounded or angular 

 ridge divides the muscular scars, which thus form two elongated depressions 

 margined on their outer sides by the prolonged basis of the dental plates. 

 The occlusor leaves a small not always clearly defined impression on either 

 side of the mesial ridge, and it is probable that the larger impression named 

 divaricator, and marked (R) in our figures 9 and 13 of 0. Michelmi, and 0. re- 

 supinata is apparently composed of two parts, the anterior, or central, being 

 the devaricator, while the other, the posterior, or lateral, which is paral- 

 lel may belong to the ventral adjustor ? 



In the dorsal valve the fissure is almost entirely occupied by a mode- 

 rately produced shelly, or cardinal process, to which were no doubt affixed 

 the divaricator muscular fibres ; the inner socket walls are sometimes some- 

 what prolonged under the shape of projecting laminae, to the extremity of 

 which free spiral arms may perhaps have been affixed, while under this 

 sheUy process, a longitudinal ridge, with a wide flattened space on either 

 side, separates the quadruple impressions of the adductor, or occlusor muscles, 

 tliese last producing two oval-shaped depressions, placed obliquely one above 

 the other, and separated by lateral ridges, branching from the central ridge. 

 Yascidar impressions and ovarian markings are often clearly observable in the 

 interior of both valves. 



Orthis resupinata has sometimes attained two inches and a-half in length by 

 rather more than three and a-half m breadth ; but the largest Scottish example 

 that has hitherto come under my observation did not exceed about one inch in 

 length, by one and a quarter in breadth. 



This sliell occurs at Gare, in Lanarkshire, at two hundred and thirty-nine 

 fatlioms lower than the " Ell Coal," and three hundred and forty-one at Raes 

 Gill. It is found also in the same county at Middlehobn and Brockley, near 

 Lesmahago; Capel Rig, East Kilbride; Ketherfield and Gallow Hill, near 

 Strathavon; and Robroyston, to the north of Glasgow. In Ayrshire, at 

 Aucheuskeigh, near Dairy; West Broadstone, Beith; Craigie, near Kilmar- 

 nock ;_ Ccssnock, near Galstouc In Dumbartonsliire at Castlecary. In Stir- 

 liii^^shire it occurs in several stages. At Balglass Bui-n, in the Campsie 

 mam-limcstoue, and Corrie Burn beds. It has also been collected in Ai'rau, 

 at Charlestown, in Eifeshne, and at Scola Burn, in Midlothian. 



^ Prior to concluding our notice of 0. resupinata, we must aUude to certain spe- 

 cimens of a ^^niall Ortliis, first discovered by Mr. Young, at Corrie Bum, andre- 



rcscnietl in our plate bv fig. 15. This little sheU was for some time considered 

 oiuil;- and myself as possibly a distinct species, but I am now dis- 

 posed to belir\(> il a^ young, or sm'all exceptional shape, or variety, of 0. 

 rcsKpinaJfi, in \\\\w\\ llio area in either valve is unusually developed; the stria- 

 tiou of the surface of the valves does not appear to differ from that of 

 Martin's shell, and cvidnicc of spines and tubercules may also be clearly 

 obsen-ed. We will, lluuTftn-e, provisionally at least, consider this small shell, 

 with widely separated beaks as an exceptional shape, or variety, of 0. 

 rcsvpttfcita. 



