898 MOLLUSC A. ARCADE. Arca. 



rior side, with the upper margin shorter, straight, and nearly 

 closed. 



Balanus Bellonii, List. Conch, t. ccvii.— Musculus striatus, Bwl. Corn. 

 277- t. xxviii. f. 15 — Arca Nose var. Don. Brit. Shells, t. clviii. f. 3. 



A. fusca, Alont. Test. Brit. Sup. 51. t. 4. f. 3. Titrt. Biv. Brit. 16? 



Southern coasts of England, and Frith of Forth, rare. 

 This species nearly resembles the last, of which it is perhaps only a varie- 

 ty. It (iiffers, however, in being narrower, the beaks more lateral, a greater 

 convexity, straighter margin, and finer decussations. 



402. A. tetragona. — Shell I'hombic, with granular decussa- 

 tions, and an oblique longitudinal rib, open at the margin. 



Lamk. Hist. vi. 37- Turt. Biv. Brit. 107- t. 13. f. 1 At the Calves, 



west of Ireland, Dr Turton. 

 " Shell |ths of an inch long, and an inch broad, of an oblique square figure, 

 tumid at the beaks, from which proceed a nearly central rib, to the angular 

 point of the opposite margin ; hinge not qviite straight, in consequence of the 

 marginal slope ; colour whitish-brown, inside white, with a purple blotch at 

 one end." Turt. 



403. A. barbata. — Shell oblong, with longitudinal bearded 

 ribs, decussated by minute striae ; the margin straight and near- 

 ly closed. 



Brown, Wern. Mem. ii. 512. t. xxiv. f. 3. — A. reticulata, Turt. Biv. Brit. 

 168. Foimd in Loch Strangfoi'd, by Dr Magee. 

 Length half an inch, breadth one inch ; shell white ; cuticle chesnut ; ribs 

 about twelve, nearij' equidistant and flat ; inside striated longitudinally, 

 margin finely crenulated. 



404. A. lactea. — Shell oblong, decussated and bearded, an- 

 gular at the anterior end, and slightly open at the margin. 



Pectunculus exiguus. List. Conch, t. cccxxxv — A. lac. Linn. Syst. i. 114L 

 —A. barbata, Penn. Brit. Zool. iv. 98. t. Iviii. f. 59.— A. lac. Mont. Test. 

 Brit. 138.— A. perforans, Turt. Biv. Brit. 1G9. t. 13. f.2,3 — Different 

 parts of the coast, but not common. 

 Length half an inch, breadth three quarters ; shell white, with a brown 

 pilose cuticle ; ribs numerous, fine, crossed bj' the wrinkles of growth ; inter- 

 stices punctured; inside white, margin plain- 



EXTINCT SPECIES. 



1. A. subacuta. Gibbose; margin an obtuse scalene triangle, deeply plait- 

 ed ; breadth greater than the length ; surface longitudinally striated.— •■S'om'^t. 

 Min. Conch, t. xliv. Upper fig. — In Chalk Marl. 



2. A. carinata Very convex parallelopipedal, anterior side flattish, sepa- 

 rated by an acute angle, truncated at nearly a right angle ; posterior side 

 rounded ; surface longitudinally ribbed — Soioer. Min. Conch, t. xhv- Lower 

 figure. — In Green Sand. 



3. A. Branderi. — Transversely elongated, rhombic, gibbose, finely decus- 

 sated ; beaks distant ; longitudinal stride strongest ; teeth numerous ; edge in- 

 ternally entire. — Sower. Min. Conch, t. cclxxvi — London Clay. 



4. A. appendiculata. — Transverse!}^ elongated, rhombic, gibbose, decussatoii ; 

 beaks rather distant ; two oblong appendages upon the space between llie 



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