MAZATLAN UNIVALVES 309 



-f- 359. PETALOCONCHUS MACROPHRAGMA, n. s. 



P. t. parvd, dextrali, dense purpureo-fuscd ; cylindraced, 

 solute spirali, marginibus spirce scepe subparallelis ; plerumque 

 glomerante, interdum solitarid ; basi ad conchas, seu alter a, ad 

 alterant, constricte adharente, s&pe erodente ; superficie rugis 

 irregularibus spiralibus hand extantibus, et riigulis increment* 

 ornatd: intus anfr. primis, etiiltimis quoad iv., apertis; medianis 

 laminatis ; lamina superior e multo ma j ore, prius conspicud, a 

 columelld extante ; primum simplici, dein angulo recto rejlexd, 

 extus carinis i-iii. quarum ii. acutissimis ; lamina inferior e 

 simplici, a columelld extante, altero juxta carinam fere attin- 

 gente ; lamina tertid minima, intercalate, inferiori pane attin- 

 gente ; laminis tenuissimis, albis, diapkanis, lineis increments 

 conspicuis; pagind inter nd maxime nitente, transversim hand 

 septato. 



The shell is of small diameter ; when growing freely taking 

 a tolerably regular spiral, like a Turritella squeezed sideways ; 

 the whirls enlarging very slowly, and resembling a winding 

 staircase. It is known when fresh by its lustrous purple-brown 

 colour and absence of pits on the surface. It occasionally eats 

 into the shell on which it grows, like Spiroglyphus, from 

 which it is known by being dextral and cylindrical. Sometimes 

 it clusters in large masses, like Bivonia glomerata. At which 

 whirl the internal laminae commence, it is difficult to say ; they 

 have been counted running through 10 whirls ; and the terminal 

 number of open whirls appears to vary. At each end they 

 commence (the large upper one first) as simple columellar 

 plaits, afterwards increasing till they fill the greater part of the 

 cavity and nearly meet. The upper one bends at right angles, 

 with two sharp keels at the corners, and a third, not always 

 developed on the upper side. The lower one goes to meet it, 

 forming with it and the columella a rectangle, only com- 

 municating with the remainder by a narrow slit. A small 

 columellar plait supports the inside base of the lower lamina. 

 The delicate texture of these laminae resembles the cup in 

 Crucibulum, &c. No transverse partitions have been seen. A 

 solitary specimen on the base of a Uvanilla unguis has at least 

 13 whirls, and measures long. '65, lat. '13, div. 6. A large 

 sp. measures lat. '23, diam. apert. *07. 

 Hab. Mazatlan ; not uncommon, on Uvanilla, Cuma, Murex 



princeps, &c. ; L 'pool fy Havre Coll. Panama ; on Margari- 



tiphora; Mus. Cuniing. 



