8 ARCA. ARK. 



white, rounded at the posterior end, elongated and a little 

 angular at the other, covered with an olive-brown skin, 

 and beset with a marginal beard of strong brown hair-like 

 byssus, with numerous nearly equidistant flat longitudinal 

 ridges which are covered with fine short thickset brown 

 hair, and crossed with numerous fine lines giving it a retir- 

 culate appearance ; the hollows at the anterior and poste- 

 rior slopes are also covered with bristles of a dark umber 

 color: beaks nearer one end, pointed and close together ; 

 inside rather glossy, with pale rays, and clouded with pale 

 chesnut ; the margin finely notched : teeth in a straight 

 line, rather small, but increasing in size as they recede from 

 the centre : length half an inch ; breadth rather more than 

 an inch. 



It differs from A. barbata, in having the margin notched, 

 and in being a little angular on the anterior side. 



A perfect specimen was found alive, adhering to an 

 oyster, from Killinchy in Lough Strangford, Ireland, by 

 Dr. Macgee of Belfast. 



4. Area Nucleus. Silvery Ark. Fig. 1, 2. 



Da Costa, pi. 15. f. 6. right hand fig. Linn. Trans, vi. 

 pi. IS. f. 3 to 6 Donovan,^ pL 63 Dorset CM,' pi. 12. f. G. 

 right-hand fig. 



"Shell somewhat triangular, slightly convex, produced on 

 one side, covered with a smooth olive-green skin, under 

 which it is white with generally some yellowish rays from 

 the beak to the margin, and mostly marked with flesh- co- 

 lor or blueish transverse bands and a few coarse ridges, 

 finely and regularly striate longitudinally : beaks small, 

 turning towards the shorter side, beneath which is an oval 

 depression; inside bright silvery : teeth in a curved line, 

 ,-ibout twenty on the elongated side, and half the number on 

 the other, separated by a triangular cavity : length half an 

 inch or more ; breadth not so much. 



On most sandy shores, v. v. 



B. With the margin very entire. 5, to 11. 



5. Area minima. Least Ark. 



Shell somewhat globular, a little produced on one side, 

 transparent, glossy, smooth or so minutely striate longitu- 

 dinally 



