MAZATLAN UNIVALVES 309 
359. PETALOCONCHUS MACROPHRAGMA, 2. S. 
P. t. parva, dextrali, dense purpureo-fusca ; cylindraced, 
solute spirali, marginibus spire sepe subparallelis ; plerumque 
glomerante, interdum solitaria ; bast ad conchas, seu altera ad 
alteram, constrictée adherente, sepe erodente ; superficie rugis 
irregularibus spiralibus haud extantibus, et rugulis increments 
ornata: intus anfr. primis, et ultimis quoad iw., apertis; medianis 
laminatis ; lamind superiore multo majore, prius conspicud, a 
columellé extante; primum simplici, dein angulo recto refiexd, 
extus curinis i-ili. quarum ii. acutissimis ; lamind inferiore 
simplici, a columella extante, altero juxta carinam fere attin- 
gente ; lamind tertid minima, intercalante, inferiort pene attin- 
gente; laminis tenuissimis, albis, diaphanis, lineis incrementt 
conspicuis ; paging internd maximé nitente, transversim haud 
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 laminz 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 laminze resembles the cup in 
Crucibulum, &e. 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, Jat. °13, div. 6°. A large 
sp. measures lat. '23, diam. apert. *07. 
Hab.—Mazatlan ; not uncommon, on Uvanilla, Cuma, Murex 
princeps, &e.; L’pool & Havre Coll— Panama; on Margari- 
tiphora; Mus. Cuming. 
