THRACIA. 37 



truncated at the posterior end, with a well-defined angle on 

 that side ; posterior dorsal margin somewhat recurved, instead 

 of sloping ; anterior dorsal margin longer : beaks less promi- 

 nent, with a slighter and less distinct excavation below them : 

 ligament rather large, but short, yellowish-white or pale brown, 

 keeping the valves asunder on the posterior side, and when 

 removed leaving a lanceolate gap : cartilage yellowish-brown ; 

 pits obliquely elongated sideways, and not projecting so far 

 inwards as in T. praztenuis ; connecting ridge at the bottom 

 thicker and less distinct : ossicle semiannular, placed as in that 

 species : other particulars the same, except that in the present 

 species the beaks only (and not the hinge) are fissured, and 

 the rib-like mark of repair in the interior is therefore wanting. 

 L. 0-6. B. 1-1. 



Yar. 1. gracilis. Shell more slender, and approaching a 

 cylindrical shape, thinner, more uniformly convex ; posterior 

 end shorter in proportion. 



Yar. 2. viUosiuscula. Thicker, and less elongated trans- 

 versely ; posterior angle more rounded, but truncated. Ana- 

 tina viUosiuscula, Macgillivray in Edinb. New Phil. Jonrn. 

 April 1827, p. 370, pi. i. f. 10,' 11. T. viUosiuscula, F. & H. i. 

 p. 224, pi. xvii. f. 4, 7. 



Monstr. Furrowed on the posterior slope, or having mis- 

 shapen valves. 



Habitat : Sandy bays in the laminarian zone ; rather 

 common. Yar. 1. Bantry Bay, and twenty miles north 

 of Unst in 86 f. Var. 2. As widely diffused as the 

 typical form, but usually in deeper water, or where the 

 supply of calcareous material is more plentiful. Fossil 

 at Belfast (Grainger) ; Kyles of Bute, and Lochgilphead 

 (Geikie) ; Coralline Crag (S.Wood). Both the typical 

 kind and the variety viUosiuscula range from Iceland to 

 the iEgean and Canaries, at depths of from 2 to 35 f. ; 

 an intermediate form has been taken by Steenstrup in 

 Iceland, by Malm on the Swedish coast, and by 

 M 'Andrew in Yigo Bay. Mr. Malm's son found the 

 variety subfossil at Uddevalla, and Sars in the newer 

 part of the glacial formation near Christiania. 



