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. Rarely 
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 id 43 a GEL S851 opi oy atl) eok Oi 
A rounded bs 3 safe EPDM, >) kw al Byes wily 
A produced ne és se rdeel alte OT ef. oT dd, 
Hah—Salango and Xipixapi; in sandy mud, 9 fm ; Cuming.— 
Keuador ; Xipixapi, D’ Orbigny.—Panama, extremely rare ; 
C. B. Adams.—Mazatlan ; very common; L’pool & Havre 
Coll._—(T. Hindsii) Guayaquil Bay, Mus. Cuming.—(T. gra- 
cilior) loc. ineog.—(T. semifusca) Mazatlan, Welchers.—(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 ecrenations 
outside, as sometimes in Gnathodon trigona.—229, 1 sp. dis- 
torted growth, gaping extremely.—230, 1 sp. mantle cut. 
