MAZATLAN UNIVALVES 305 



The longest spire measures '44 by "14; the adult, long. 1*9, 



diam. apert. "5. ' 



Hob. Mazatlan ; extremely rare, on Chamse and Spondyli ; 



L'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 solutb spiralis, mmqnam turritelloidea : Operculo rubro, 

 solidiore, extus parum foliaceo, intus cicatrice corrugatd, ple- 

 rumque vix concentrice lirulatd, s&pe umbonatd. 

 (Diagn. carent.) Proc. Zool. Soc. 1847, p. 156, no. 258 : Fig. 

 Moll. An. p. 82, no. 3 ; "operculum rudimentary, small, 

 (Pspiral.)" H. fy A. Ad. Gen. vol. i. 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- 

 culum of V. glomeratus, Phil, as the concentric elements 

 present a very 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, as afterwards defined by Dr. Gray, "operculum small, 

 rudimentary," applies better to what little is known of Petalo- 

 conchus, one species of which inhabits the Mediterranean. 



355. PBlVONIA CONTOETA, n. S. 



?S. t. minor e, rubidd, cancellatd ; anfr. nucleosis iii. Itevibus, 

 elevatis ; normalibus irregulariter uno super alio decliviter 

 et spiraliter incumbentibus, marginibus scepe vix parallelis : 

 plerumgrue solitarid, conchis variis basi totd tenue adkaerente ; 

 sculptura ut in A. centiquadro ; aperturd rotundatd. 

 Comp. Vermetus glomeratus, Rouss. in CMnu HI. Conch, pi. 2, 



f. 2 (1843). Wee. in Zeit. f. Hal. 1847, p. 178, no. 4 : do. 



1850, p. 165, no. 14. C. B. Ad. Pan. Shells, p. 216, no. 323. 

 6. Panama ; attached by a small part of the spiral por- 



