MAZATLAN UNIVALVES 197 
may adopt both forms of marginal insertion at different periods 
of growth, which is contrary to our observation in the young 
of other species ; or (2) that a different creature, belonging by 
its plates to one genus, puts on the exact appearance of a species 
in another, even to the very conspicuous seales, which are for- 
bidden by the diagnosis to Lepidopleurus, and are also striated 
which is equally forbidden in Lophyrus. Under either suppo- 
sition, it would appear that we have not yet attained a correct 
knowledge of the principles of division to be adopted in this 
most remarkable family. Long. ‘13, lat. ‘08, alt. .025. 
Hab.— Mazatlan; on Spondylus calcifer, extremely rare ; 
‘pool Col. 
Tablet 904 contains the perfect specimen, kindly presented 
by T. Nuttall, Esq. Ofits habitat nothmg is certainly known. 
The Avicula to which it adhered was exactly like the Mazat- 
lan shells; and there were also found on its surface several 
specimens of Septifer Cumingianus, and two of a species of 
Cecum, both identical with those from Mazatlan. 
256. ? LEPIDOPLEURUS BEANII, 7. s. 
2D. t. ovata, elevatd (ad angulam 110°), fusco-olivaced, seu 
albido ceruleoque maculata ; valvis intermediis valde mucrona- 
tis, interstitiis curvatis, marginibus subrotundatis ; valvd pos- 
tica depress, excavatd, mucrone minimo, superiore; jugo et 
areis lateralibus indistinctis : superficie toté granulis instructd, 
et punctulis minimis confertim ornata: valvarum lobis magnis, 
curvatis, sinu planato; marginibus acutis, fissuris circiter x. 
valvis terminalibus, und in utroque latere, intermediis ; imargint- 
bus externis prominentibus ; limbo pallir piluloso, spinulis par- 
vis, erectis, planatis. 
Differs from L. Mac-Andrei in being longer, with the valves 
mucronated and rounded at the extremities, the posterior 
being hollowed out, and the rest much elevated, without con- 
spicuous middle portion or lateral areas. The only perfect 
specimen found (on Acmea fascicularis) displays no trace of 
solid scales. The account of the interior is taken from a large 
central and small anterior valve supposed to belong to this 
species from their agreement in other respects. Long. *23, 
lata No ole: U4. 
Hab.— Mazatlan ; extremely rare; L’pool Col. 
Tablet 905 contains the perfect specimen off Acmea, and 
_ the two valves. 
