MAZATLAN UNIVALVES 341 



doubt, simply because the Mazatlan shells were not numerous 

 enough to oppose, while they did not confirm, his arrangement. 

 While in Cerithiopsis, the sculpture is tolerably constant, in 

 these, as in the British species, the rows of nodules and the 

 convexity of the spire outlines vary in arrangement on the 

 same specimen according to age. The basal sculpture does 

 not appear in the young shell. 



391. TRIFOEIS ALTEENATUS, C. B. Ad. 



Pan. Shells, no. 207, pp. 158, 310. 



Shell with the first 7 whirls transparent, light horn-coloured ; 

 the first two smooth, then with numerous radiating lines, 

 crossed by first one, then two, then three spiral ridges, which 

 gradually become nodulous. Then of a purplish brown, for 

 many whirls with two rows of stout, lighter-coloured nodules. 

 Between these a fine line appears which gradually develops 

 into first a smaller and at last an equal sized row of nodules. 

 Below are one keel on the periphery and two on the base. 

 The colour changes to whitish on the lower portion of the later 

 whirls. The shell is comparatively large, a fragment measur- 

 ing "08 across. A specimen with 9 normal whirls measures 

 long. '17, long. spir. '14, lat. '055, div. 20. 

 Hob. Panama, 5 sp. C. B. Adams. Mazatlan ; portions of 



8 sp. off Spondylus ; L'pool Col. 



Tablet 1614 contains a very young sp., a perfect adolescent 

 one, and a fragment of an adult. 



392. TEIFOEIS INCONSPICUUS, C. B. Ad. 



Pan. Shells, no. 208, pp. 159, 310. 



Shell in sculpture closely resembling T. alternatus, but much 

 smaller ; with the nuclear whirls shorter in proportion, light- 

 coloured only at the apex, developing the nodules and the dark 

 colour earlier and more gradually. Colour dark throughout ; 

 the last two whirls with 3 complete rows of nodules. Anterior 

 spiral ribs larger, that on the periphery sometimes slightly 

 sculptured. A specimen very perfect at the mouth but with 

 the apex deficient, measures long. '15, long. spir. '115, lat. '05, 

 div. 18. 



Hob. Panama ; 16 sp. under stones near low water mark ; 

 C. B. Adams. Mazatlan ; 12 sp. more or less complete, off 

 Spondylus and Chama ; L'pool Col. 



