IT 



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 (PI. XV. Fig. 6.) 



T. transversa, subfalcata, ventricosa, tenuis, lucida vivide margaritacea, 

 concentrice undulata plerumque nuda ; apice ad trientem anteriorera ; latere 

 antico rotundato ; latere postico attenuate, truncate, subascendente, utroque 

 extremitate lineis radiantibus remotis striato. 



Shell very thin and fragile, transparent, extremely bright pearly, 

 somewhat conspicuously undulated concentrically, destitute of epi- 

 dermis and of radiating striae except at the posterior lip, 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 re- 

 curved, the surface more pearly and free from striae ; O.floridanum 

 is more venlricose, much more attenuated posteriorly, the surface 

 more dead, and the stria3 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 stramineA in- 

 duta, rugis remotis radiata, apicibus vix antemcdianis, tumidis ; latere antico 



3 



