190 
CONCHIFERA. 
A RCA. 
13. Pectunculus semi-truncattjs. —The Semi-truncated 
Pectunculus. 
P. semi-truncatus. Portlock, Geo. Sur. p. 429, pi. 34, 
fig. 7. 
Nearly orbicular, a little oblique; moderately convex, with 
a slight truncation behind ; hinge-lino very moderately curved, 
extending a short distance behind the beaks, but more length- 
end in front; surface smooth ; margin slightly depressed. 
The Silurian Limestone, Desertcreat, Tyrone, Ireland. 
14. Pectuncuxus ambiguus.— The Ambiguous Pecfcun- 
culus. 
P. (?) ambiguus . Portlock, Geo. Sur. p. 430, pi. 34, f. 11. 
Sub- orbicular; compressed; beaks small and pointed; 
hinge-line straight and short; both sides equally rounded ; 
surface smooth, with very faint lines of growth. 
The Silurian Limestone, Desertcreat, Tyrone, Ireland. 
Genus X.—MACRODON.— Lt/cett. 
Shell equivalvo, transverse, inequilateral, sub-quadrate, 
somewhat ventricose ; hinge-lino nearly parallel; beaks small, 
placed near to one end, remote, separated by a pretty broad 
area; hinge with six obliquely-parallel linear teeth in the 
right valve, situated near the anterior extremity, the inner¬ 
most tooth stretching transversely nearly the entire length 
from the hinge-line ; theso teeth are received into correspond¬ 
ing cavities formed for their reception in the opposite valve ; 
base, or ventral margin, provided with a hiatus for the pas¬ 
sage of the byssus, and producing a corrugation in tho edge of 
the valves ; two muscular impressions in each valve, tho an¬ 
terior one furnished with a prominent ledge projecting from 
the side of tho shell, the posterior one expanded and in¬ 
distinct. 
I. Macrodon rugosus. — Tho Rough Macrodon, pi. 
LXI *** fig. 20. 
M. rugosus . Murchison, Geo. Cheltenham, 2d Ed. p. 99, 
pi. 5, fig. 5. 
Transversely elongated, its width being a little more than 
twice its length ; surface and marginal outline flexuous ; both 
extremities somewhat truncated ; numerous elevated concen¬ 
tric lines of growth traverse its surface, with many strong, 
prominent, radiating, narrow ribs crossing tho intervening 
spaces : basal line undulating. 
In the Oolite, top of Leckhampton and Critchley Hills, and 
near Minchinhampton. 
Genus XI.—ARC A .—L in nee us. 
Shell transverse, equivalve, sub-quadrate, inequilateral, 
ventricose ; hinge-line straight, generally angular at both ex¬ 
tremities, sometimes a little rounded ; hinge with numerous 
small, close-set, notched teeth, for the most part increasing in 
size as they diverge from the beaks ; umbones remote, sepa¬ 
rated by a wide area, on which the ligament is spread in cross 
rows; surface mostly longitudinally ribbed; two lateral, 
distant, muscular impressions in each valve; ligament ex¬ 
ternal. 
L A rca Eastnori. —The Eastnor Area, pi. LXXVIL 
fig. 27. 
A. Eastnori. Murchison, Silur. Sysfc. Part II. pi. 20, f. 1. 
Shell thick, transversely ovate, extremely convex; beaks 
short, nearly central; muscular impressions deep; tho poste¬ 
rior ono considerably so; hinge-line nearly rectilinear ; teeth 
small, upright, and numerous, and slightly divergent; length 
upwards of half an inch; breadth nearly an inch. 
In the Curadoc Sandstone, at Golden Grove, Llandoilo, and 
also in Eastnor Park. 
2. Arca appendiculata. —The Appendaged Area, pi. 
LXXVIL fig. 17. 
A. app&ndiexdata. Sowerby, III. p. 135, pi. 270, fig. 3. 
Transversely elongated, rhombic, gibboso ; beaks somewhat 
distant and incurved ; two oblong appendages on tho hinge 
area, between the beaks; whole surface pretty closely decus¬ 
sated ; the longitudinal ridges frequently furcated, and deeply 
intercepting the lines of growth ; teeth not very numerous; 
internal margin toothed. 
The London Clay, Barton Cliff. 
3. Arca Branderi. —Brandor’s Arca, pi. LXXVIL f. 10. 
A. Branderi . Sowerby, III. p. 135, pi. 270, figs. 1, 2. 
Transversely elongated, gibbose ; beaks remote ; hinge-line 
straight, each side terminated by an auriform process ; an ob¬ 
tuse ridge emanates from the beak, terminating near the front 
in tho lower side ; spaco between the beaks plain, except hav¬ 
ing tlireo or four impressed strim, which aro more perpendicu¬ 
lar than usual; surface very finely decussated ; teeth numer¬ 
ous ; external edgo entire. 
The London Clay, Barton Cliff. 
4. Arca cancel lata. —The Cancellated Arca, pi. 
LXXVII. fig. 18. 
A. cancellata . Sowerby, V. p. 115, pi. 473, fig. 2. Arcites 
cancellatus , Martin, Pet. Derby, pi. 44, fig. 7. 
Transversely elongated, its width being nearly double its 
length ; beaks somewhat produced, and nearly touching; pos¬ 
terior side rounded; anterior side nearly parallel, defined by a 
keel ; marginal sinus short and deep, whole surface coverod 
with longitudinal and transverse strim, producing an elegant 
cancellated appearance. 
Carboniferous Limestone, Derbyshire. 
5. Arca pulchua. —The Splendid Arca, pi. LXXVIL 
fig. 29. 
A. pulchra. Sowerby, V. p. 115, pi. 473, fig. 3. 
Elongated, transversely ovate, its width nearly twice its 
length, depressed; anterior side considerably impressed, ob¬ 
liquely truncated ; beaks approximating, whole surface with 
close, uniform, fine striro. 
Great Oolite, Ancliff, Wiltshire. 
G. Arca quaduisulcata. —Tho Four-furrowed Area, pi. 
LXXVIL fig. 30. 
A . quadrisulcata. Sowerby, V. pi. 473, fig. 1. 
Convex, its width twice its length ; anterior side truncated 
and defined by a keel, furnished with four deep, well-defined 
furrows; posterior side small, rounded ; margin furnished with 
a large marginal sinus ; surface longitudinally striate, and 
crossed by lines of growth, producing a rugged aspect. 
