U^t-s hells and Mussels 63 



seen in the larvcal condition of many species, and 

 this is part of the evidence that the bivalves have 

 degenerated from a higher primitive type. The 

 conical white foot has a deep central groove, through 

 which a dark green byssus is spun. The porcelain- 

 lined interior of the shell bears large oval impressions 

 of the muscles, and the inner margin is marked with 

 a series of indentations, those of one valve fitting into 

 those of its companion. The Noah's Ark occurs on 

 all our coasts, at all depths. 



There are four other native species : one, the 

 Hebridean Ark (J., 'pectiincidoides), with thinner 

 shell, finer sculpture, and the left valve larger than 

 the right, is found chiefly in deep water (35 to 100 

 fathoms) round Shetland and the Hebrides. 

 Another, the Milky Ark {A. lactea), has 

 valves of equal size, their white colour 

 partially hidden by the brown velvety 

 epidermis. The animal is not much given 

 to burrowing, but chiefly attaches its short horny 

 byssus-threads to the inside of old shells and to 

 rock-crevices between 15 and 25 fathoms, on all our 

 shores. 



The Dog - Cockle or Comb - shell (Pectiinculus 

 glycmieris) has an appearance quite distinct from 

 any other native species, and requires very little 

 description to enable one to identify it. The shell 

 is nearly circular, of a dull yellowish - white colour, 

 irregularly marked with zigzag lines and conical 

 patches of dark red. The epidermis forms a thick 

 dark brown velvet, but from most specimens that 

 are washed up tlie beach this is all rubbed off'. On 

 specimens dredged up from the deeper water it 

 5 



