76 On the Absorption of the Internal Structure of Shells. 



the Murices, for instance, destroy external spines, and the Pur- 

 puree wear away the walls of their apertures. Cones and Olives 

 reduce the thickness of the septa between the whorls of their 

 shells. A species of Harpa {H. articulata) is said to absorb only 

 the central part of the septa, so as to leave a slit between the 

 cavities of the different whorls. Other Mollusks carry this ab- 

 sorption of parts of their shells to a still greater extent. The 

 Neritidas and Auriculidee dissolve all the internal spiral column, 

 as shown in the accompanying specimens. 



I was not, however, aware until within the last few days, 

 that this alteration of internal structure prevailed in Stbastoma, 

 Lucidella, Trochatella, Helicina, and Proserpina, — the latter 

 genus non-operculated ; possibly the fact has been observed by 

 others. 



"While examining a broken young shell of H. maxima Sowb. 

 from Jamaica, I noticed the absence of the internal spiral 

 column and septa. The smoothness of the interior of the shell 

 beneath the apex, and particularly along the line of the sutures 

 of the upper whorls, precluded the idea of accidental injury to 

 the shell. On opening more individuals of the same and other 

 species of Helicina, and of Lucidella, Trochatella, and Stoastoma, 

 I found the same reduction of the interior of the shells, — the 

 outer wall of the last whorl (continued to a small extent be- 

 yond and in the rear of the operculum) only remaining. A 

 similar alteration was also observed in Proserpina. 



I exhibit specimens of H. maxima Sowb., albolabris C. B. Ad., 

 palliata C.B.Ad., Hollandi C.B. Ad., and neritella Lam., L. aureola 

 Gray, T. Tanhervillii Gray, and pzdchella Gray, and also of S. 

 pisum C. B. Ad., P. pulchra C. B. Ad. (said to be var. of P. lin- 

 guifera Jonas), nitida Sowb., pisum C. B. Ad., globulosa (Orb ), de- 

 pressa (Orb.), and bidentata C.B.Ad., in which this alteration of 

 structure can be seen. It will be noticed that the edge of the 

 septa, where the absorption was discontinued, is rounded off, as 

 if to protect the animal from the injury which a sharp or un- 

 even edge would be likely to produce. That this absorption 



