82 SAXICAVID.E. 



towards the siphonal extremity: colour whitish: epidermis 

 light brownish-yellow, more or less puckered : margins rounded 

 on the anterior side, nearly straight in front, either curved or 

 bluntly truncated on the posterior side; dorsal edges gently 

 sloping on each side, the posterior one being three or four 

 times as long as the other : beaks small and blunt, inflected, 

 and inclining considerably to the anterior side : ligament yel- 

 lowish-brown, proportionally longer than in the last species : 

 hinge-line slightly curved : hinge-plate broad and thick, exca- 

 vated externally to receive the ligament, so as to form in some 

 specimens an elongated ledge or process, which is reflected 

 outwards and callous in younger shells ; it is occasionally also 

 excavated (but slightly) internally : teeth often wanting ; but 

 when they occur, the right valve has a very small erect car- 

 dinal, closely interlocking between two others in the left valve : 

 inside porcelain -white and glossy : pallial scar exhibiting in 

 dead and fossil specimens a few spots of different sizes, which 

 are indistinct in fresh specimens : muscular scars more con- 

 spicuous, triangular. L. 0-6. B. 1-4. 



Var. 1. arctica. Shell more angular, and having distinct 

 ridges; beaks less worn; teeth usually more developed: this 

 variety never burrows in stone, but is attached by a byssus. 

 Mya arctica, Linn. S. N. p. 1113. S. arctica, F. & H. i..p. 141, 

 pi. vi. f. 4-6. 



Var. 2. minuta. Shell smaller, and having prickly ridges : 

 this is the younger or immature state of the first variety. 

 Solen minutus, Linn. S. N. p. 1115. 



Var. 3. prazcisa. Shell smaller, abruptly truncated close to 

 the beaks at the anterior end. Mytilus proedsus, Mont. Test. 

 Brit. p. 165, t. 4. f. 2. 



Var. 4. pholadis. Shell gaping widely in front, and wedge- 

 shaped. Mytilus pholadis, Linn. Mant. Plant, p. 548. 



HABITAT : On every part of our coast, from the Shet- 

 land to the Channel Isles, where there is limestone, 

 chalk, or new-red sandstone, all of which this species 

 excavates. It usually inhabits the lowest verge of spring- 

 tides, and the Laminarian zone; but Mr. Peach pro- 

 cured live specimens from a rock perforated by them in 



