54 



Rounded, more oblique and rather flatter than the two 

 preceding, the posterior ventral margin rather enlarged ; the 

 auricles nearly equal, with serrated striee ; ribs angular, finely 

 tuberculated. This species differs from the last in shape, 

 in the greater angularity of the ribs, and their rugosity. The 

 varieties are reddish brown, with zigzag white marks ; purple, 

 variegated; and pale orange, variegated with brown and 

 white. Great Britain and Mediterranean. 



25. P. EXASPERATus. (pL xviii. f. 183 to 186.) nobis. 



T. rotundato-sub-quadrata, sub-ventricosa, scabra; auriculis 

 magnis, scabroso-striatis ; costis octodecim, sub-angulatis, 

 angulis serratis, lateribus serratim striatis, interstitiis scabroso- 

 striatis ; colore vario. Long. 0. 95. ; lat. 0. 55. ; alt. 1. ; poll. 



In the character of the ribs this species resembles the last, 

 they are, however, rather thicker, the shell is more ventricose, 

 less oblique, and has broader auricles. The variations in 

 colom* are similar to those of the last species. Mediterranean. 



26. P. BiFBONS. (pi. 12. f. 9. 10.) Lam. Anim. s. Vert, 

 vi. p. 164. 



T. rotundata, antice miniis expansa, scabra ; auriculis latis, 

 quadratis, striatis, posticis paululum minoribus ; costis novem 

 paululum elevatis, rotundatis, duplicatis vel triplicatis ; inter- 

 stitiis latis, radiatim irregulariter costatis ; colore valvse sinis- 

 tree pallide purpureo, costis fuscis nigricantibus, prope um- 

 bones maculatis, valvee dextree albo, costis rubris, intus pur- 

 pureo ; variat purpureo. Long. 2. 30. ; lat. 0. 75. ; alt. 2. 20. ; 

 poll. 



A nearly equivalve, nearly equilateral shell with nine 

 slightly raised distant ribs, and smaller ribs at the sides and 

 in the interstices ; the margin is shaded with light purple, and 

 the ribs are of a deep blackish red colour, excepting near the 

 umbones, where they are spotted. It also occurs with a 

 stronger shade of purple, and the ribs are not so distinctly 

 lineated. 



27. P. Patagonicus. (pi. xiii. f. 60.) iiCm^. Zool. Journ. 

 T. rotundata, sub-ventricosa, sub-leevi ; auriciilis striatis, 



obtusis, anticis majoribus ; costis 34, sub-quadratis, elevatis, 

 IcGvibus ; colore pallide fulvo, costis fulvis rubescentibus. 

 Long. 2. 40. ; lat. 0. 80. ; alt. 2. 40. ; poll. 



Rounded, rather ventricose, nearly smooth ; the auricles 

 striated, obtuse, the anterior the larger; the ribs, 34 in 

 number, are nearly square, elevated and smooth ; the colour is 

 pale fawn, the ribs reddish brown. 



Patagonia and the Straits of Maaralhaens. 



