392 BRITISH PALAOZOIC FOSSILS. [ LamMELLIBRANCHIATA. 
PTERONITES SUBRADIATUS (Sow. Sp.) 
Ref. and Syn.= Avicula (Monotis Bronn.) subradiata Sow. Geol. Trans. 2nd Series, Vol. V. t. 54. f. 1. 
Sp. Ch.—Uongitudinally aliform or subtrigonal, very slightly convex ; beaks very small, inconspicuous, 
near the very narrow rounded anterior end, from which the margin is gradually and slightly arched to the 
respiratory angle, which is obtusely rounded ; posterior end very broad, the margin slightly sigmoid, subtruncate, 
with a slight obliquity ; hinge-line nearly as long as the shell; cardinal angle nearly rectangular; a triangular 
space extending from the beak along the hinge-line, rather less than half the width of the posterior margin, 
marked with very fine, nearly obsolete, radiating striz, rest of the surface radiated with coarse, rugged, slightly 
unequal, narrow ridges, separated by spaces about equalling them in width (about five or six in the space of two 
lines at ten lines from the beak); whole surface crossed by delicate, subregular, concentric, elevated, narrow 
plicee, varying from a line to half a line apart on the middle of the shell. Length one inch six lines, propor- 
18 
tional length of hinge-line 7%, width from beak at right angles to hinge-line 4, length of anterior end 4, width 
1009 
50 
of posterior end about > 
100* 
Position and Locality—Common in the slate of S. Petherwin. 
Genus. AVICULOPECTEN (M°Coy). 
Gen. Char.—Inequivalve, more or less inequilateral; straight, or slightly extended obliquely towards the 
posterior side; anterior ear flattened, smaller than the posterior, sharply and deeply defined, with a deep notch 
in the right valve between it and the body of the shell for the passage of the byssus; posterior ear slightly 
pointed, extending about as far as the margin of the shell, defined or not; ligament confined to a narrow facet 
along the hinge-margin ; no medial cartilage pit ; muscular impression and pallial scar as in Pecten. 
It was only on seeing the fine suite of fossils from the dark limestone of Lowick, Northumberland, recently 
presented by the Rev. Mr Jenkins to Prof. Sedgwick, and now in the collection of the University of Cambridge, 
that I recognised the characters by which the great bulk of the so-called Pectens of the middle and upper 
Paleozoic rocks are distinguished from the true Pectens of the more recent formations and present sea. In the 
present fossils the posterior ear is largest, thus differing in an external character from Pecten and approaching 
Avicula, an affinity greatly increased by the internal structure exposed by the Lowick (and some of Mr Griffith’s 
Irish) specimens, shewing that there is no mesial ligamentary pit beneath the beak as in the former genus, but, 
as in the latter, the ligament is confined to the hinge-margin; while in general form and little or no obliquity 
of the shell, the resemblance this genus bears to Pecten is very striking. 
The discovery of this character fixes the zoological place of numerous carboniferous shells constantly 
varying hitherto in the systems between Pecten, Avicula, and Meleagrina. 
AVICULOPECTEN GRANOSUS (Sow. Sp.) 
Ref. and Syn.= Pecten granosus Sow. Min. Con. t. 574. f. 2. 
Sp. Ch._—Truncato-orbicular, compressed, gently convex; hinge-line nearly equalling the width of the shell; 
anterior ear shorter than the anterior margin, strongly defined from the body of the shell by its abrupt de- 
pression, and a small sigmoid sinus in the margin, besides a narrow, nearly smooth space at the junction with 
the body of the shell destitute of radiating ridges; posterior ear broadly falcate, undefined; cardinal angle 
slightly acute, reaching as far as, or slightly beyond, the posterior margin of the shell; surface of the middle 
and posterior wing uniformly covered with thick, obtusely nodular, slightly irregular ridges, separated by deep, 
smooth, narrow concave spaces, with often a much smaller slightly tubercular ridge between some of the pairs of 
larger ; four or five ridges on the anterior ear beyond the smooth space above mentioned ; in the middle of the 
shell at one inch from the beak about seven to ten strize in the space of three lines. Length and width nearly 
equal. varying from one to nearly three inches, proportional length of posterior ear ;;;, of anterior ear 7. 
Position and Locality.—Rare in the slate of S. Petherwin. 
