MAZATLAN UNIVALVES 311 



The outside presents the appearance of a set of saucers, but 

 is more distinctly spiral even than in Aletes. When viewed 

 inside by transmitted light, the size of the last whirl is not 

 seen, owing to the intercalation of the external ridges. 



Tablet 1510 contains the only specimen found. 



(e.) Operculum minimum, corneuin, tenue, Jlavidum, diapha- 

 num ; extus marginihus convexis, laminis concavis solute axiyn 

 extantem concentrice, sjihspiralitei' ascendentibus, haudfoliaceis, 

 suherenulatis ; intiis marginem versus convexum ; cicatrice 

 parvci, circulari, lirulis extantibus concentricis circiter xii. 



One imperfect specimen alone was found. It most resembled 

 Vermetus proper. The ridges appeared principally concentric, 

 but here and there running into each other, as tliough spirally. 

 The tiny saucers were piled on each other outside, so that the 

 external outline was convex, iyter repeated examinations, it 

 sprang from the forceps, and was lost ia the meshes of the 

 carpet. 



Tablet 1511 contains a sketch. 



(f.) Operculum tenue, rubra - aurantium, haud regulare ; 

 extus (plerumque dense incrustata) concavum, maxime intus 

 umbonem, marginibus concentricis irregulariter vix extantibus ; 

 intu^ lineis concentricis pluribus vix conspicuis ; umbone latiore, 

 valde extante ; margine lata, corneo, plus minusve convexo ; 

 muse, cicatr. circulari, partem interiorem occupante. 



Six opercula were found, probably belonging to Bivonia 

 contorta, whose reddish orange colour is cousi)icuous against 

 the very thick coraUiuous incrustations which generally cover 

 the outer part. The growth is irregular, but slightly concave 

 externally, except inside the boss where it is very dpep. The 

 margin is also irregular, and of a light horn colour. It closely 

 resembles the operculum of Bivonia glomerata, Phil., which is 

 generally similarly incrusted. 



Tablet 1512 contains 2 opercula, shewing variations in gi'owth, 

 with the thick incrustation removed from one of them. 



(g.J Operculum solidius, dense rufo-purpureum, ; extus con- 

 vexum, marginibus foliaceis tenuibus, angulo fere recto extanti- 

 bus : intus concavum ; margine haud interrupto, valde rotunda to, 

 convexo, angustiore, diaphand ; cicatr. muse, haud conspicue 

 definitd, lirulis concentricis viii. con-ipicuis ; umbone maxima 

 extante, subspirali. 



