56 MAZATLAN BIVALVES 



occasionally rather large : the margin is generally thickened 

 in that part, as in Donax. The growth of the teeth is very 

 irregular. The posterior tooth is rugosely crenated in adoles- 

 cent shells, but in very young ones this is not the case, and in 

 old specimens it is frequently obsolete : in this state it becomes 

 the T. semifulva of Menke. If I have rightly affiliated the 

 young specimens, ('03 across) the creature begins as a sub- 

 orbicular body, with very swollen and pointed umbos, white 

 with a purplish tinge, the anterior part swollen. There are 

 then only 2 teeth ; an enormous anterior lateral, and a small 

 posterior, nearly at right angles. Gradually these lessen in 

 comparative size, while the cardinal teeth are formed one by 

 one, and the creature assumes a trigonal and afterwards a 

 transverse form. The youngest which is undoubtedly T. radi- 

 ata, (about '22 across,) has not yet attained its full compliment 

 of teeth, and does not display crenations. In colour, the shell 

 is generally more or less brown ; either in a diffused hue, or in 

 in rays of endless variety, sharp or mottled ; straight or zigzag ; 

 with the umbos almost always tinged with purple at the tip, 

 with or without a white patch, bifurcate or irregular. Harely 

 however most of the forms take a uniform dull white, without 

 purple tips. The shell generally forms irregular ridges of 

 growth, as in Dione aurantia. 



A transverse specimen measures long. 1 '72, lat. 2 ' 23, alt. 1 ' 14. 



A thick 1*88, 2*3, 1'34. 



A rounded 1*57, 1*82, 1. 



A produced 171, 1*97, 1*11. 



Sal). Salango and Xipixapi ; in sandy mud, 9 fm ; Cuming. 



Ecuador; Xipixapi, D'Orbigny. Panama, extremely rare ; 



C. S. Adams. Mazatlan ; very common ; L'pool fy Havre 



Coll.(T. Hindsii) Guayaquil Bay, Mus, Cuming. (T. gra- 



cilior) loc. incog. (T. semifusca) Mazatlan, MelcJiers. (T. 



intermedia) California, Mus. Cuming. 



Tablets 217 235 contain specimens arranged to. shew varia- 

 tions in form. 217, 3 sp. very transverse. 218, 3 sp. rather 

 less. 219, 3 sp. somewhat rounded, tumid. 220, 4 sp. very 

 regularly rounded. 221, 1 sp. ventral line flattened. 222, 

 1 sp. posterior part produced. 223, 1 sp. anterior part pro- 

 duced. 224, 3 sp. gibbous. 225, 3 sp. subtriangular. 226, 

 4 sp. and a valve, subtriangular, produced anteriorly. 227, 

 1 sp. extreme form, flat, gaping, 228, 1 sp. with crenations 

 outside, as sometimes in Grnathodon trigona. 229, 1 sp /dis- 

 torted growth, gaping extremely. 230, 1 sp. mantle cut. 



