MAZATLAN UNIVALVES 305 
The longest spire measures “44 by ‘14; the adult, Jong. 1°9, 
diam. apert. °5. 
ifab.—Mazatlan ; extremely rare, on Chamz and Spondyli; 
DT’ pool Col. 
Tablet 1486 contains 1 extremely young and 1 adult spire.— 
1487, 2 sp. irregular portion commencing: one has cemented 
to itself the valve of a Donax.—1488, the adult specimens, on 
Chama frondosa (Mexicana.) 
Section B. BIVONIA, Gray. 
Testa solute spiralis, nunquan turritelloidea: Operculo rubro, 
solidiore, extus parum foliaceo, intus cicatrice corrugatd, ple- 
rumgue vix concentricé lirulata, sepe umbonaté. 
(Diagn. carent.) Proce. Zool. Soc. 1847, p. 156, no. 258 :—Fig. 
Moll. An. p. 82, no. 3; “operculum rudimentary, small, 
(Pspiral.)”"—H. & A. Ad. Gen. vol. 1. p. 358; “operculum 
red, convex in the middle, with a thin, flat margin, and a 
central muscular scar.” 
The name is retained for the typical species quoted by Dr. 
Gray in default of description. It was natural that Bivon and 
Philippi should not be agreed in their account of the oper- 
eulum of V. glomeratus, Phil. as the concentric elements 
present avery spiral appearance, and the outside is almost 
always incrusted with coralline &c. which, even if removed, 
probably carries off the foliated’ surface along with it. The 
genus, a3 afterwards defined by Dr. Gray, “‘opereulum small, 
rudimentary,” applies better to what little is known of Petalo- 
conchus, one species of which inhabits the Mediterranean. 
305. P BIVONIA CONTORBTA, 7. Ss. 
* 
?B.t. minore, rubida, cancellaté ; anfr. nucleosis iii. levibus, 
elevatis ; normalibus trregulariter uno super alio decliviter 
et spiraliter incumbentibus, marginibus sepe vie parallelis : 
plerumque solitarid, conchis variis basi totd tenue adharente ; 
sculpturd ut in A. centiquadro ; aperturd rotundata. 
Comp. Vermetus glomeratus, Rouss. in Chénu Ill. Conch. pl. 2, 
f. 2 (1843).— Mike. in Zeit. f. Mal. 1847, p. 178, no. 4:—do. 
1850, p. 165, no. 14.—C. B. Ad. Pan. Shells, p. 216, no. 323. 
Hab.—Panama ; attached by a small part of the spiral por- 
