416 BRITISH PAL/EOZOIC FOSSILS. {[Bracuropopa. 
of about 100° with the lateral margins; mesial lobe defined in most specimens near the beak by two sulci rather 
stronger than the rest. Receiving valve very gibbous, almost equally arched from the beak to the middle of the 
front margin, so that its depth is greatest in the middle; beak narrow, prominent, much incurved over the 
hinge-line; cardinal area rather wide, triangular, beaks approximate ; dental lamellie diverging at a very small 
angle (10°); surface of both valves covered with very numerous, subequal, very much branched ridges, which, 
when the surface is well preserved, seem smooth, flat, and separated by extremely narrow slightly marked sulci, 
those of the sides and mesial fold exactly similar and equal, ten or twelve in the space of six lines on all parts 
of the shell (eight or nine in the same space in the var. crassa); but when partially decorticated, the sulci 
become very much deeper and larger, and from the primary sulci being much more deeply grooved than all the 
subsequent subdivisions of the primary ridge the radiation in this state seems very strongly fasciculated ; at 
nine lines from the beak there may commonly be counted nine strize on the mesial fold, which unite into three 
at six lines from the beak, and subsequently into one; about nineteen ridges on each side, every two or hree of 
which are separated by much stronger sulci, and blend into seven or eight primary ridges near the beak; some 
of the extreme lateral ones being simple from this length to the adult size; the ridges increase in number by 
frequent branchings, but do not change in size. Proportions of young specimen one inch wide, proportional 
length of receiving valve 5, of entering valve ;%, height of area ;;;, width of hinge-line 5%, depth of entering 
valve %, depth of receiving valve ;;;, depth of sinus in front margin =. Adult specimen, width three inches, 
proportional length of receiving valve ;%, of entering valve 7, height of area ;;,, width of hinge-line ;;, depth of 
entering valve *, of receiving valve ,;;, depth of front sinus ;j. 
Although the large specimens with nearly smooth, well-preserved surface, and very close flat ribs, so 
common in the dark limestone of Killymeal, Dungannon, which I figured formerly under the name of S. plani- 
costata as above, seemed to have no affinity with the smaller specimens from other localities, which agree 
perfectly with Phillips’s original S. duplicicosta, with coarse sulci separating the strongly branched and fasciculated 
ridges, yet I now suspect the difference of striation is due to the amount of preservation of the shell, and the 
difference in form to age; and (although I may be wrong) I would now venture to unite them, from the study of 
abundance of variously preserved specimens of all ages in the collection from Derbyshire. M. d’Orbigny has 
suggested the union of the S. planicostata and the S. crassa of De Koninck, and a specimen from Derbyshire, 
perfectly according with the latter species, is so perfectly identical with the present fossil in all points save the 
flatness of the mesial ridge, and a somewhat greater width and depression (proportional length 4, depth of 
both valves ;,), that I have no doubt it should only be classed as a variety of S. duplicicosta also. 
Position and Locality—Very common in the great scar carboniferous limestone of Derbyshire; not very 
uncommon in the black lower carboniferous limestone of Isle of Man; rare in the dark lower carboniferous 
limestone of Kendal; not rare in the limestone of Lowick, Northumberland. 
SPIRIFERA FASCICULATA (J/'Coy). PI. 3. D. fig. 25. 
Ref—M ‘Coy, Ann. Nat. Hist. 2nd Series, Vol. X. 
Desc.—Longitudinally very broad-ovate ; length and width nearly equal, depressed ; hinge-line only half 
the width of the shell; cardinal area rather more than one-third higher than wide; lateral margins horizontal, 
front margin abruptly raised into a defined, obtusely angular sinus; entering valve depressed slightly, and 
gently convex on the sides ; mesial fold very strongly defined by a sulcus and depression of the surface on each 
side; convex, very obtusely angular, but not very prominent, having three ridges near the beak, which dicho- 
tomise into six as they approach the margin; each side has about seven to nine flat, smooth ridges near the 
beak, most of which branch into two or three as they approach the edge; the intervening sulci very narrow ; 
receiving valve more convex, moderately gibbous along the middle, sloping rapidly towards the side margins ; 
mesial hollow strongly defined, very deep, narrow, flattened near the beak, widening, and receiving about two 
plaits on each side of the middle one as it reaches the front sinus. Width of average-sized specimen nine 
lines, proportional length of receiving valve 4, of entering valve , width of cardinal area °, width of sinus in 
100 
