190 



CONCHIFERA. 



Arca. 



13. Pectuncclus semi-truncatus. — ^The Semi-truncated 

 Pectunculns. 



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-line 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. Pectunculus ambiguus. — The Ambiguous Pectun- 

 culns. 



P. (?) ambiffuus. 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.— Zy««. 



Shell equivalvc, transverse, inequilateral, sub-quadrate, 

 somewhat ventricose ; hinge-line 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*he anterior extremity, the inner- 

 most tooth stretching transversely nearly the entire length 

 from the hinge-line ; these 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 the edge of 

 the valves ; two muscular impressions in each valve, the an- 

 terior one furnished with a prominent ledge projecting from 

 the side of the shell, the posterior one expanded and in- 

 distinct. 



1. JIacrodon RtiGosus. — The Rough Macrodon, pi. 

 LXI.*^-* fig. 26. 



M. rugosiis. 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 the intervening 

 space.; ; basal line undulating. 



In the Oolite, top of Leckhampton and Critchley Hills, and 

 near !Minchinhampton. 



Genus XI. — ARCA. — Linnaeus. 



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. 



1. Arca Eastnori. — The Eastnor Arca, pi. LXXVII. 

 fig. 27. 



A. Eastnori. Murchison, Silur. Syst. Part II. pi. 20, f. 1. 



Shell thick, transversely ovate, extremely convex ; beaks 

 short, nearly central ; muscular impressions deep ; the poste- 

 rior one 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, Llandeilo, and 

 also in Eastnor Park. 



2. Arca appexdiculata. — The Appendaged Arca, pi. 

 LXXVII. fig. 17. 



A. appendiculata. Sowerby, III. p. 135, pi. 276, fig. 3. 



Transversely elongated, rhombic, gibbose ; beaks somewhat 

 distant and incurved ; two oblong appendages on the 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 Clifl". 



3. Arca Branderi. — Brander's Area, pi. LXXVII. f. 16'. 

 A. Branderi. Sowerby, III. p. 135, pi. 276, 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 the lower side ; space between the beaks plain, except hav- 

 ing three or four impressed striie, which are more perpendicu- 

 lar than usual ; surface very finely decussated ; teeth numer- 

 ous ; external edge entire. 



The London Clay, Barton Clifi". 



4. Akca cancellata. — The Cancellated Area, pi. 

 LXXVIL 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 covered 

 with longitudinal and transverse striae, producing an elegant 

 cancellated appearance. 



Carboniferous Limestone, Derbyshire. 



5. Arca pdlchra. — The Splendid Arca, pi. LXXVII. 

 fig. 29. 



A. pulohra. 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 striaj. 



Great Oolite, Ancliff, AViltshire. 



6. Arca quadrisulcata. — The Four-furrowed Arca, pi. 

 LXXVII. 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 furuished with 

 a large marginal sinus ; surface longitudinally striate, and 

 crossed by lines of growth, producing a rugged aspect. 



