MAZATLAN UNIVALVES 31k 
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, corneum, tenue, flavidum, diapha- 
num ; extus marginibus convexis, laminis concavis solute axim 
exiantem concentricée, subspiraliter ascendentibus, haud foliaceis, 
subcrenulatis ; tintus marginem versus convexum ; cicatrice 
parva, 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 though spirally. 
The tiny saucers were piled on each other outside, so that the 
external outline was convex. After repeated examinations, it 
sprang from the forceps, and was lost in the meshes of the 
carpet. 
Tablet 1511 contains a sketch. 
Gf.) Operculum tenue, rubro-aurantium, haud regulare ; 
extus (plerumque dense incrustata) concavum, maxime intus 
umbonem, marginibus concentricis trregulariter vie extantibus ; 
intus lineis concentricis pluribus viz conspicuis ; umbone latiore, 
valde extante ; margine lato, corneo, plus minusve convexo ; 
muse. cicatr. circulari, partem interiorem occupante. 
Six opercula were found, probably belonging to Bivonia 
contorta, whose reddish orange colour is conspicuous against 
the very thick corallinous incrustations which generally cover 
the outer part. The growth is irregular, but slightly concave 
externally, except inside the boss where it is very deep. The 
margin is also irregular, and of a light horn colour. It closely 
resembles the operculum of Bivonia glomerata, P/il., which is 
generally similarly incrusted. 
Tablet 1512 contains 2 opercula, shewing variations in growth, 
with the thick incrustation removed from one of them. 
(g.) Operculum solidius, dense rufo-purpureum ; extus con- 
vexum, marginibus foliaceis tenuibus, angulo fere recto extanti- 
bus : intus concavum ; margine haud interrupto, valde rotundato, 
convexo, angustiore, diaphand ; cicatr. muse. haud conspicué 
definita, Uirulis concentricis vill. conspicuis ; umbone maxime 
ext ante, subspiralt. 
