390 Gould's Descriptions of Shells 
and even purple posteriorly ; cicatrices broad and chalky white, 
very distinct ; siphonal sinus semicircular ; tooth on the right valve 
short, subulate, recurved. 
Longitud. diam. one and one eighth inches; vertical, seven 
eighths of an inch ; transverse, six eighths of an inch. 
Inhabits Guaymas. Lieut. Green. 
This is the analogue of P. rocellaria Lk. and scarcely to be dis- 
tinguished from it; but its posterior half seems to decline much 
more rapidly; the radiating folds are much less marked ; on the 
left valve is a single tooth; on the right are two, less developed ; 
the pallial scar, between the siphonal sinus and anterior muscular 
scar, is not sinuous, and grows larger; interior not always tinted, 
nor cicatrices chalky. 
OSTEODESMA NITIDUM (Pl XV. Fig. 6.) 
T. transversa, subfaleata, ventricosa, tenuis, lucida vivide margaritacea, 
concentric? undulata plerumque nuda; apice ad trientem anteriorem ; latere 
antico rotundato; latere postico attenuato, truncato, subascendente, utroque 
extremitate linei diantibus remotis striato. 
Shell very thin and fragile, transparent, extremely bright pearly, 
somewhat conspicuously undulated concentrically, destitute of epi- 
dermis and of radiating strie except at the posterior tip, where it 
is very thin and pale straw color; beaks at the anterior third, ele- 
vated; disks tumid ; anteriorly rounded ; posterior portion ascend- 
ant, gradually narrowing till its truncated tip becomes about half 
as elevated as the valve at the beaks. 
Longitud. diam. seven eighths of an inch; vertical and trans- 
verse three eighths of an inch. 
Inhabits Santa Barbara. Lieut. Green. 
Differs from O. hyalinum in being more inequilateral, more Te 
curved, the surface more pearly and free from stris ; O. floridanum 
is more ventricose, much more attenuated posteriorly, the surface 
more dead, and the striæ much more remote from each other. 
LUTRARIA VENTRICOSA. 
Lutraria ventricosa Gould ; Proc. B. S. N. H. Nov. 1851. IV. 89. 
T. tenuis, fragilis, ventricosa, ovato-cuneata, alba, epidermide stramineà e 
duta, rugis remotis radiata, apicibus vix antemedianis, tumidis; latere antico 
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