Bracuropopa.] UPPER PALAZOZOIC MOLLUSCA. 431 
Position and Locality—Common in the carboniferous limestone of Isle of Man; rare in the carboniferous 
limestone of Kendal, Westmoreland, 
Sprrirera (Martinia) syMMeETRICA (Phill) 
Ref.—S. id. Phill. Geol. York. Vol. II. t. 10. f. 13. 
Dese.—General form rhombic, moderately gibbous, rather depressed; hinge-line slightly more than half the 
width of the shell; beaks approximate; cardinal angles narrow, rounded; anterior lateral margins long, and 
nearly straight or obscurely sigmoid, from being slightly convex near the cardinal angles and slightly concaye 
towards the front, which is broad and produced; lateral margins not horizontal, but slightly sigmoid and 
abruptly raised in the front, at an angle of 105°, into a large rounded sinus, usually slightly notched in the 
middle; entering valve subtrigonal, depressed, moderately and evenly convex in the posterior half; greatest 
depth about the middle ; profile slightly sigmoidal from the raising of the front margin, which produces a faint 
indication of a broad, rounded, mesial ridge, seldom perceptible except at the margin; often a faint, very narrow, 
mesial sulcus, obscurely marked from the margin nearly to the beak, after six or seven lines from the beak; the 
sides abruptly arch downwards to the lateral margins, leaving the mesial ridge elevated and obscurely defined ; 
beaks small, moderately prominent; cardinal area distinct. Receiving valve rhombic, moderately and evenly 
gibbous ; posterior lateral margins and anterior lateral margins nearly straight ; the beak, the front and the 
cardinal angles, narrow, obtusely rounded ; greatest depth at one-third the length from the beak, from whence 
the profile arches almost semicircularly to the apex, which is rather small, narrow, and much incurved, while 
from the same point the profile is more gradually arched to the produced front, filling the broad mesial sinus in 
the margin, which produces scarcely any indication of a corresponding broad mesial hollow, except in very old 
specimens; but there is usually a very faint, narrow, mesial sulcus resembling that in the entering valve, 
extending nearly to the beak; sides moderately convex. Shell thin, very minutely fibrous under the lens, 
indistinctly marked with longitudinal, subregular, thread-like strize, separated by much wider flat spaces (four 
to five in three lines at an inch from the beak), which appear to be seen through the substance of the shell from 
the interior, where they are much more strongly marked. Width of average specimens one inch seven lines ; 
65 80 
proportional length of entering valve =, length of receiving valve >, length of hinge-line %, width of cardinal 
10 a5 
area 72, width of sinus in front margin *%, depth thereof =, depth of entering valve ,3, depth of receiving 
valve =. 
This beautiful and very distinct species varies very little in its characters from the above description; it is 
remarkable for the nearly regular, rhombic outline of the receiving valve, for the broad, often minutely notched 
sinus in the front margin, producing scarcely any distinct mesial ridge, and for the strong, filiform, subregular, 
distant, radiating lines from the beak to the margin of the interior, often appearing on the external surface. 
Two specimens differ remarkably (one from Lowick and one from Derbyshire), by the hinge-line being only ;;,, 
as compared to the width; and one of them by the length nearly equalling the width, and the shortness of the 
beak of the receiving valve, so strongly approximates to the S. decora, that I suspect additional experience may 
unite these species. Both differ from all the varieties of Spirifera (Martinia) glabra by the strong, thread- 
like, subregular, internal ridging from the beak to the margin. 
Position and Locality —V ery common in the lower carboniferous limestone of Lowick, Northumberland ; 
rare in that of Derbyshire; rare in dark lower carboniferous limestone of Isle of Man; rare in the carboniferous 
limestone of Kendal; var. in carboniferous limestone of Beith, Ayrshire. 
[rasc. 11. ] 3K 
