MAZATTAN BIVALVES 57 



White or very faintly streaked variety. Tablet 231 contains 

 4 sp., 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, 3 sp. 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. 241, 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. 



f 84. TRIGONA HUMILIS, n. s. 



T. t. juniore ovali, wmbonibus appressis ; ? adultd parvd, 

 subtrigond, margine ventrali excurvd, latd; umbonibus non 

 incurvis, parum prominentibus ; Icevi, vel striis incrementi exili- 

 bus : fused, intws mdculo pu rp ur eo dente postico ; 

 dent, lat., antico brevi, solido, postico longo, exili ; card. 2-3 

 parvis ; lined pallii a margine remotd, sinu parvd, subascen- 

 dente ; ligamento celato, brevi ; epidermide tenui. 



This shell might be taken for the young of Dione chionsea, 

 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 long. '02, lat. '035, alt. '015. 

 The largest *11, '16, '07. 



H$b 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. 



