MAZATLAN UNIVALVES 2a. 
that of U. olivacea in being sharper at the outer edge, more or 
less granular on the ridges (in the young shell over the whole 
surface), and with the small interior rib beginning within the 
other. Its apex fits against the periphery of the shell. A very 
finely grown spreading specimen measures long. 1°2, lat. 1°7, 
(without the palme 1°36,) div. 80°. The largest, conical sp. 
measures long. 1°73, lat. 1°8, (without the palme 1°56,) div. 60°. 
Hab.—Mazatlan; extremely abundant; Z’pool Col.—S. W. 
Mexico, very fine, P. P. C. 
Tablet 1097 contains 5 sp. different ages, spreading variety.— 
1098, 6 sp. do. ordinary form.—1099, 3 do. conical.—1100, 3 do. 
sculpture clear.—1101, 1 do. curiously mended after fracture.— 
1102, 1 do. upper part eaten away, leaving the liver chamber 
with the whirls disunited, like Vermetus.—1103, 1 do. curiously 
crowned with Coralline.—1104, 1 do. covered with Alge.— 
1105, 2 do. bored by young Lithophagi.—1106, 1 do. with large 
Annelid, of which an operculum is added from a full grown 
sp. on Spondylus, distinguished by the two large and two 
small branching stag’s-horn processes. 
Specimens shewing base. Tablet 1107 contains 3 sp., palme 
broad, expanded.—1108, 1 do. more distant.—1109, 1 do. very 
narrow.—1110, 2 do. palme numerous.—1111, 3 do. irregularly 
developed.—1112, 1 do. with labium narrowed by Sabellarie, 
which, along with Bryozoa contending with Vermetide for 
superiority, have covered the base. —1113, 1 do. with base 
curiously rounded.—1114, 1 do. mouth blistered.—1115, 1 do. 
with attachment of Calyptreid outside the basal edge of the 
mouth. 
Tablet 1116 contains 2 sp. with the opercula in situ, one 
young, the other adult.—1117, 6 opercula. 
Genus TROCHUS, Linn. 
Trochus, Linn., auct., pars :—--Phil. Hand. Conch. p. 210. The 
ancient name is retained for these, the ordinary forms of the 
original genus. 
Ziziphinus, Leach :—H. & A. Ad. Gen. vol. i. p. 421 :—Mon. 
in Proc. Zool. Soc. 1851, p, 163. 
289. Trocuus veRsicotor, Mike. 
Zeit. f. Mal. 1850, p. 172, no. 32. 
? = Ziziphinus Californicus, A. Ad. Proc. Zool. Soc. 1851, p. 168, 
no. 52. 
