218 MAETIN F. WOODWARD. 



second specimen, however, showed that P. Beyrichii, like 

 the two other species mentioned above, possessed a fairly 

 stout though somewhat small operculum attached to the foot 

 by a large circular lobe (figs. 3 and 4). We are, however, 

 still unable to determine whether the first specimen had lost 

 its operculum during its free life, or if it had been born 

 without one. Judging from the presence of the opercular 

 lobe, I should be inclined, in spite of its small size, to suggest 

 that the operculum had been present, but accidentally lost 

 either through disease, or mishap, early in life, 



The operculum (fig. 4) is nearly circular in outline, 

 measuring, in the largest specimen, 14*5 mm. in diameter; in 

 character it is trochiform, consisting of about twenty closely 

 coiled whorls, strongly marked Avith line of growth. It is 

 apparently composed solely of dark brown horny (chitinous) 

 matter, and for its size is very thick and sti'ong, retaining its 

 thickness quite to the margin. 



The mouth of the shell from which the operculum was taken 

 measured 40 mm. in transverse and 30 mm. in vertical dia- 

 meter. Hence it will be seen that the operculum can be of 

 very little use in closing the aperture, and thus protecting 

 the retracted animal; it may, however, be of some service in 

 protecting the upper surface of the foot from mechanical 

 injury which might be caused by the rubbing of the shell 

 when the animal was fully extended, since under these con- 

 ditions the shell rests, as in the Trochidae and Turbinidas, 

 directly upon the operculum. 



Compared with the opercula of P. Quoyana and P. Adan- 

 soniana, the operculum of P. Beyrichii appears to most 

 nearly i*esemble that of the first-named species, although 

 Dall in his description does not mention what is such a 

 striking feature in P. Beyrichii, the thickness of the oper- 

 culum. In P. Adansoniana the operculum is very much 

 larger and thinner, and still more closely resembles the 

 opercula of the Trochidfe. 



The small size of the operculum in two of the three speci- 

 mens, and its absence in the third, suggests that this organ 



