Bracniopvopa. | UPPER PALAZEOZOIC MOLLUSGCA. 449 
small hinge-line; the striation somewhat resembles that of the O. connivens, but is more regular; it differs from 
that of the O. resupinata in wanting the peculiar gradual dilatation of certain parts of some of the strize before 
the occurrence of a spine-like projection, and the remarkable abrupt diminution of the swollen ridge beyond the 
spine; the spines in the present species being difficult to see, and never producing those elongate, drop-like 
marks of that species. The front is sometimes very faintly sinuate in the middle, and occasionally with an almost 
imperceptible wave towards the entering valve. The elongate subtriangular form, and general depression of the 
valves, as well as the difference noted in the striation, separate it from both the O. resupinata and O. connivens. 
Position and Locality—Extremely common in the lower carboniferous limestone of the Isle of Man, very 
common in the carboniferous shales of Beith, Ayrshire. 
ORTHIS RESUPINATA (Mart. Sp.) 
Tief. and Syn.= Anomites resupinatus Martin, Pet. Derb. t. 49. f. 13 and 14= Terebratula resupinata Sow. 
Min. Con. t. 325 = Orthis id. de Koninck, Foss. Bel. t. 13. f. 9. 
Desc.—Transversely oval, one of the long sides slightly truncated by the hinge-line, which is of moderate 
length ; cardinal angles obtuse, sides semielliptically rounded; front wide, broadly arched ; lateral and front 
margins in one plane in young specimens, raised with a very shallow wide wave towards the entering valve in 
old ones. Entering valve deeper than the receiving one, broadly and nearly uniformly convex, flattened at the 
posterior lateral margins ; beak obtuse, prominent, incurved to the level of the hinge; profile regularly arched, 
greatest depth at about one-third from the beak; in some specimens an exceedingly faint, narrow, mesial con- 
cavity, extends nearly from the beak to the front margin. Receiving valve very slightly longer than the entering 
one; beak small, slightly prominent; rostral portion for about six lines from the apex rather tumid, sloping 
rapidly to the sides and front margin, so that beyond the above length the sides and front of this valve are 
generally flattened, or sometimes even slightly concave either in the middle or at the sides. Surface of both 
valves uniformly covered with close, fine, slightly unequal, thread-like strize (averaging twenty in two lines at 
six lines from the beak), at irregular alternating distances some of the strize rather rapidly increase in thickness 
and prominence, giving origin to a slight spine-like projection at the distal end, beyond which the ridge either 
dichotomises, or is continued of abruptly diminished size; concentric imbrications of growth small, moderately 
numerous ; surface under the lens very minutely granulo-punctate. Average width one inch six lines, propor- 
tional length of receiving valve 7%, of entering valve %, width of hinge-line 4, height of cardinal area ;{;, depth 
of receiving valve ;%, depth of entering valve 3; width occasionally two inches six lines, proportional depth of 
both valves varying from = to =. 
This species is more depressed and more finely striated than the O. connivens, and has the entering valve 
shorter and very much less tumid; it is wider and considerably more tumid near the beaks than the O. J/i- 
chelini, and from both it is distinguished by the beautiful peculiarity of certain of the striz, being irregularly 
swollen and abruptly truncated, resembling the appendages to ermine fur. One crushed Isle of Man specimen 
is almost as gibbous in the entering valve and with as distinct a sinus in the front as the O. connivens, but 
having the striation of O. resupinata. The cardinal area of the entering valve is small, and placed in the plane 
of the lateral margins, the triangular opening is flanked by two very strong dental lamelle, diverging with its 
sides at 95°, and forming deep slits in the cast ; in the middle of the apex of the opening is a very small bifid 
rostral tooth, the sides of which diverge at 35°, from which a slight mesial septum extends a short way from 
the beak. 
Position and Locality.—Small specimens in the black carboniferous shale of Craige, near Kilmarnock ; very 
rare in the dark lower carboniferous limestone of Lowick; very common in the lower carboniferous limestone of 
the Isle of Man; beautifully marked specimens are common in the carboniferous limestone of Malham Moor ; 
not uncommon in the lower carboniferous limestone of Derbyshire; very small specimens in the carboniferous 
limestone of Glasgow. 
