192 



CONCHIFERA. 



CVCVLLJEX. 



Transversely elongated, cylindrical and convex ; anteriorly 

 rouuded ; obliquely truncated posteriorly ; teeth nearly in a 

 straight line. 



The Silurian Rocks, Tyrone, Ireland. 



21. Arca No;e. — Noah's Area. 



A. NooB. Brown, Recent Conch. Brit. p. 8, pi. 33, figs. 

 1, 2, 3. 



Transversely oblonp, sub-rhomboidal ; ligamentary area 

 wide ; beaks remote at their points, and rather prominent ; 

 surface decussated with fine longitudinal and transverse striae ; 

 base with a central hiatus between the valves. 



The Coral Crag, Sutton. 



22. Arca sub-truncata. — The sub-truncated Arca. 



A. suh-truncata. Portlock, Geo. Sur. p. 427, pi. 34, fig. 1. 



Transversely ovate, convex ; beaks nearly central, slightly 

 produced ; hinge-line a little curved ; anterior side obliquely 

 sub- truncated ; base a little rounded. 



The Silurian Rocks, Tyrone, Ireland. 



23. Arca regularis. — The Kogular Area. 



A. reyulai-is. Portlock, Geo. Rep. p. 427, pi. 34, fig. 2. 



Almost semicircular transversely ; hinge-line nearly straight ; 

 teeth slightly circular ; beaks central ; sides rounded ; surface 

 smooth. 



The Silurian Rocks, Tyrone, Ireland. 



24. Arca dissimilis. — The Dissimilar Arca. 



.4. dissimilis. Portlock, Geo. Rep. p. 428, pi. 34, fig. 5. 



Obliquely oval, convex ; anterior side narrow ; posterior 

 side broad ; slightly truncated obliquely ; hinge-line straight, 

 with the teeth oblique to the line ; those behind the beaks 

 in a line a little curved ; those in front few ; beaks very 

 obtuse. 



The Silurian Rocks, Tyrone, Ireland. 



25. Arca obliqua. — The Oblique Arca. 



A. oUiqua. Portlock, Geo. Sur. p. 429, pi. 34, fig. 6. 



Obliquely transverse ; ovate ; anteriorly" short and rounded, 

 with an oblique sub-truncation behind ; beaks placed very 

 near the posterior side ; teeth slightly oblique to the hinge-line. 



The Silurian Rocks, Tyrone, Ireland. 



26. Arca lactanea. — The Whitish Arca. 



A. lactanea. Wood, Mag. Nat. Hist. 1840, p. 232, pi. 

 13, fig. 3. 



Transversely oblong-ovate ; beaks approximate ; surface 

 with fine longitudinal striae, decussated with numerous strong 

 lines of growth ; internal margin destitute of crenulations ; 

 cardinal teeth vertical, becoming gradually inclined towards 

 the extremities of the hinge-line. 



lu the Red Crag, Walton, and the Coral Crag, Sutton. 



27. Arca raridentata. — The Few-toothed Arca. 



A. raridentata. Wood, Mag. Nat. Hist. 1840, p. 232, pi. 

 13, fig. 4. 



Rhomboidal, rather tumid ; anterior side short and rounded ; 

 posterior larger, obliquely sub-truncated above, and round- 

 ed below ; the basal line undulous ; beaks large, obtuse ; teeth 

 much inclined externally, and with a plain space on the car- 

 dinal area below the beaks — three on the shorter side set at 

 an angle of 45° with the hinge-line, and three nearly hori- 

 zontal ones on the longer side ; surface with fine, longitudinal 

 .striae, decussated by elevated lines of growth. 



In the Coral Crag, Sutton. 



Genus XII. — CUCULL/EA. — Lamarck. 



Shell sub-equivalve, trapeziform, or sub-quadrate ; ex- 

 tremely ventricose ; beaks distant, separated by a flat area, 

 on which the external ligament is placed ; two muscular 

 impressions in each valve ; the anterior one is elevated into 

 a sharp-edged plate or ledge, projecting from the side of the 

 shell ; posterior muscular impression fiat and indistinct ; hinge 

 rectilinear, with a series of angular, somewhat irregular teeth, 

 set in a straight line, very small near the umbones, larger and 

 more oblique towards both extremities ; outside covered by an 

 epidermis. 



1. CocuLL.EA antiqua. — The Ancient Cuculhea, pi. 

 LXXVllI. figs. 8, 9. 



C. antiqua. Murchison, Silur. Syst. pt. II. p. G02, pi. 3, 

 figs. 1 h and 12 a. 



Shell transversely ovate, smooth, rather convex ; posterior 

 side larger than the anterior, and acutely angular ; internal 

 lamina longitudinal ; umbones rather obtuse ; length about 

 three-eighths to half an inch ; breadth from half an inch to 

 three-quarters. 



Found in the lowest beds of the Old Red Sandstone, at 

 Horeb Chapel, Felindre-on-the-Teme, Wales. 



2. CucDLL^A ovATA. — The Ovato Cucullaja, pi. LXXVIII. 

 fig. 4. 



C. omta. Murchison, Silur. Syst. pt. II. p. 602, pi. 3, 

 fig. 1 2 b. 



Shell transversely ovate, and nearly convex ; umbones 

 placed near the anterior side ; interior lamina longitudinal ; 

 both sides gradually rounded ; length one inch and an eighth ; 

 breadth one inch and a-half. 



Found in the lowest beds of the Old Red Sandstone, at 

 Horeb Chapel, Wales. 



3. CucuLL.EA Cawdori. — Cawdor's Cucullsea, pi. 

 LXXVIII. fig. 15. 



C. Cawdori. Murchison, Silur. Syst. pt. II. p. 602, pi. 3, 

 fig. 11. 



Shell transversely oval, convex, nearly smooth ; anterior 

 side rounded ; posterior side obliquely truncated ; umbones 

 rather acute and nearly central, from whence a rounded ridge 

 extends to the posterior angle of the margin, with oblique in- 

 ternal laminae. 



Found in Upper Silurian Rocks at Freshwater East, Pem- 

 brokeshire. 



4. CucuLL.«A GLABRA. — The Smooth Cuculloea, pi. 

 LXXVIII. figs. 1, 2. 



C. glabra. Sowerby, I. p. 151, pi. 57. 



Rhomboidal, slightly ventricose, its width about a fourth 

 more than its length ; anterior angle obtuse ; posterior edge 

 of the front rounded ; hinge area with four divergent furrows ; 

 beaks somewhat incurved ; whole surface with fine longi- 

 tudinal striae, which are decussated by numerous lines of 

 growth ; hinge-line finely striated ; teeth deeply striated. 



Upper Greensand, Warminster and Lime ; the Lower 

 Greensand, Petersfield. 



5. CucuLL^A CARINATA. — The Keeled CucuUaea, pi. 

 LXXVII. fig. 41. 



C. carinata. Sowerby, III. p. 9, pi. 207, fig. 1. 



