luo 
THE GEOLOGIST. 
striaj, which increase in nvunber by intercalations, or bifurcations, at variable 
distances from the beaks ; and at intervals the striae themselves increase in 
thickness and prominence, giving birth to small hollow spinose asperities, or 
thread-like tubidar 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 shell-structure is also perforated by innumerable canals, of which 
the exterior orifices, in the shape of mimite punctures, cover the entu'e surface 
of the shell. 
In the interior of the ventral valve, the dental plates extend to some distance 
along the bottom of the shell, and between these a small rounded or angular 
ridge divides the muscular scars, which thus form two elongated depressions 
marghied on tlieir 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 tliat the larger impression named 
divaricator, and marked (R) in our figures 9 and 13 of 0. Micheliiii, and 0. re- 
sicpinata is apparently composed of two parts, the anterior, or central, being 
the devaricator, wliile the other, the posterior, or lateral, which is paral- 
lel may belong to the ventral adjuster ? 
In the dorsal valve the fissm-e is almost entirely occupied by a mode- 
rately produced shelly, or cardinal process, to which were no doubt aifixed 
tlie divaricator muscular fibres ; the inner socket walls are sometimes some- 
wliat prolonged under the shape of projecting lamiiiEE, to the extremity of 
which free spiral arms may perhaps have been aifixed, while luider 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 ridges, branching from the central ridge. 
Vascular impressions and ovarian markings are often clearly observable in the 
interior of both valves. 
Orthis resiipinata has sometimes attained two inches and a-half in length by 
rather more than three and a-half in breadth ; but the largest Scottish example 
tliat has hitherto come under my observation did not exceed about one inch ia 
length, by one and a quarter in breadth. 
This shell occurs at Gare, in Lanarksliire, at two hundred and thirty-nine 
fatlioms lower than the " Ell Coal," and three hundi-ed and forty-one at Raes 
Gill. It is found also in the same county at Middlehohn and Brockley, near 
Lesmahago ; Capel Eig, East Kilbride ; Netherfield 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 ; Gessnock, near Galston. In Dumbartonshire at Castlecary. In Stir- 
lingsliire it occurs in several stages. At Balglass Burn, in the Campsie 
mam-Hmestone, and Corrie Burn beds. It has also been collected in Arran, 
at Charlesto\^^l, in Eifeshire, and at Scola Burn, in Midlotliian. 
Prior to concluding our notice of 0. resnpinafa, we must allude to certain spe- 
cimens of a small Orthis, first discovered by Mr. Young, at Corrie Burn, and re- 
presented in our plate by fig. 15. This little shell was for some time considered 
by Mr. Young and myself as possibly a distinct species, but I am now dis- 
posed to believe it a young, or small exceptional shape, or variety, of 0. 
resifpitiata, in which the area in either valve is unusually developed ; the stria- 
tion of the surface of the valves does not appear to differ from that of 
Martin's shell, and evidence of spines and tubcrcules may also be clearlv 
obsen'cd. We wUl, therefore, provisionally at least, consider this small shell, 
with widely separated beaks as an exceptional shape, or variety, of 0. 
resupinata. 
