56 MAZATLAN BIVALVES 



occasionally ratKer large : the margin is generally tliickened 

 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 affliated 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 bj^ 

 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 irregvdar. Earely 

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

 purple tips. The shell generally forms ii-regular ridges of 

 growth, as in Dione aurantia. 



A transverse specimen measures long.\'7% lat.2'23, alt. I'li. 



A thick „ „ „ 1-88, „ 2-3, „ 1-34. 



A rounded „ „ ,, 1'57, ,, 1'82, ,, 1. 



A produced „ „ „ l'7l, „ 1"97, „ I'll. 



Hab. —Salango and Xipixapi ; in sandy mud, 9 fm ; Cuming.— 



Ecuador; Xipixapi, 2)'0;Vj?(7wy.— Panama, extremely rare ; 



C. B. Adams. — ]\Iazatlan ; very common ; L'pool Sf Havre 



Coll.—(T. Hindsii) Guayaquil Bay, ^lus. Cuming.— (T. gra- 



cilior) loc incog.— (T. semifusca) ^Mazatlau, Melckers. — (T. 



intermedia) California, Mus. Cuming. 



Tablets 217 — 235 contain specimens arranged to sliew 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 Gnathodon trigona. — 229, 1 sp. dis- 

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



