308 MAZATIAN UNIVALVES 



Tablet 1497 contains a sp. '08 across, on Pisania insignis, 

 containing 4 regular sinistral whirls, like Planorbis spirorbis. 

 It is smooth, and may be the same species as the above, the 

 diameter being about the same ; but the colour is much deeper. 

 Sab. Mazatlan ; extremely rare ; L'pool Col. 



358. PPBivoNiA , sp. ind. (b.) 



Tablet 1498 contains a scarcely spiral white shell wandering 

 like an Annelid over a Byssoarea solida. The texture however 

 appears Molluscan. It has two very stout rounded keels on 

 the back. The same sp. displays the attached valve of Hip- 

 ponyx serratus. 

 Sab. Mazatlan ; extremely rare ; L'pool Col. 



GENUS PETALOCCmCHUS, Lea. 



Animal ignotum. 



Testa extus " Bivonia" simillima ; intus, transversim raris- 

 sime septata ; anfractibus medianis, laminis elongatis spiralibus 

 varie dispositis, cameram scepe pcene secantibus ; plerumque 

 duabus tnajoribus, tertid, quasi plied columellari, minore. 



Operculum [specierum duarum] parvum, tenuissimum, come- 

 um, Iceve, subplanatum, mx spirale. 

 H. C. Lea, Trans. Am. Phil. Soc. 1843, vol. ix. p. 229. Woodw. 



Man. Moll. pt. 3, p. 462. Monogr. in Proc. Zool. Soc. 



July, 1856. 



The remarkable internal structure in many species of Ver- 

 metidae, appears to have been overlooked by English naturalists, 

 although it was observed many years ago by Lea in a fossil 

 species. Three species in my collection from different seas 

 having presented clear difference of external structure, I had 

 proposed for them the name of Macrophragma, which is now 

 retained for the typical, though small, Mazatlan* species. Mr. 

 Cuming having most obligingly allowed me to open several of 

 his specimens, nine species have been found, which are des- 

 cribed in the Proc. Zool. Soc. They cannot be distinguished 

 externally from Bivonia, which even as species they most 

 closely resemble. The shell has somewhat the same relation 

 to Vermetus that Nerinsea has to Cerithium, 



