MAZATLAN UNIVALVES 305 



The longest spire measm-es "44. by '14; the adult, long. 1'9, 



dia^n. apert. '5. 



Hah. — Mazatlan ; extremely rare, on Chama; and Spondyli ; 



L'pool Col. 



Tablet 1486 contains 1 extremely yonng 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, nnnquam turritelloidea : Operculo rubra, 



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



rumque vix concentrice lirulatd, sape umhonata. 



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



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



(Pspii-al.)"— ^. S)' 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, "opercidum small, 

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

 conchus, one species of which inhabits the Mediterranean. 



355. PBlVONIA CONTOETA, 71. S. 



?B. t. minore, ruhidd, cancellatd ; atifr. nucleosis iii. Icevibtt^, 

 elevatis ; tiormalibus irregulariter uno super alio decUviter 

 et spiraliter incumhentibus, marginihus scepe vix parallelis : 

 plerumque solitarid, conchis variis basi totd tenue adhcerente ; 

 sculpturd ut in A. centiqtiadro ; aperlurd rotundatd. 



Comp. Verraetus glomeratus, Rouss. in Chinu III. Conch, pi. 2, 

 f. 2 (1843).— 3/Xt. in Zeit. f. Mai. 1S47, p. 178, no. 4 :— do. 

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

 Sab. — Panama ; attached by a small part of the spiral per- 



