MAZATTAN BIVALVES 57 
White or very faintly streaked variety.—Tablet 231 contains 
4sp., regular form, various ages.—232, 1 sp. flattened, round- 
ed.—233, 1 sp. produced ventrally.—234, 1 sp. transverse, 
gaping posteriorly.—235, 1 sp. very transverse. 
Tablets 236—241 are arranged to shew transitions of colour ; 
but many of the specimens are also remarkable for form.— 
236, 3sp. white, with very faint brown markings.—237, 3 sp. 
brown rays narrow, interrupted.—238, 2 sp. very narrow and 
numerous rays.—239, 3 sp. with broad, sharp rays.—240, 3 sp. 
brown predominant.—24l, 3 sp. diffused brown. 
Tablet 242 contains 6 pairs, extremely young, which pro- 
bably belong to this species. 
Tablet 243 contains 2 young valves, which may belong to 
the transverse white variety. 
84. TRIGONA HUMILIS, 7. s. 
T. t. guniore ovali, tinbonibus appressis; ?adulta parva, 
subtrigond,- margine ventrali excurvd, lata&; umbonibus non 
wncurvis, parum prominentibus ; levi, vel striis incrementi exili- 
bus: fusca, intus maculo purpureo dente postieo; 
dent. lat., antico brevi, solido, postico longo, exili ; card. 2-8 
parvis ; lined pallii a margine remota, sinu parva, subascen- 
dente ; ligamento celato, brevi ; epidermide tenut. 
This shell might be taken for the young of Dione chionea, 
but for the entire absence of spirally recurved umbos ; it differs 
also from the shells supposed to be the young of T. radiata in 
the flatness of the umbos and the comparatively small size of 
the teeth. Not being able to affiliate it with any satisfaction, I 
have been compelled to describe it provisionally. In the 
youngest stage only the lateral teeth are prominent, and the 
shape is almost oval; then the anterior tooth turns round and 
makes a cardinal, and afterwards the other cardinal teeth ap- 
pear. There seems always a stain of purple inside the ligament. 
When adult, it becomes subtrigonal, with the dorsal margins 
rather straight. Neither lunule nor area are defined. 
The youngest specimen measures Jong. °02, Jat. ‘035, al#. °O15. 
The largest ou GY ao Aiea sam ne 
Hab—Mazatlan ; rather uncommon, in Spondylus and Chama 
washings ; L’pool Col. 
Tablet 244 contains 2 pairs united, and 4 pairs of valves of 
different ages. 
