283 



OX THE OCCURRENCE OF IXTERXAL SEITA IX GLYPTOSTOMA 

 NE WhEl'Ji YANUM. 



By G. K. (iciiE, F.Z.S. 



ReaA lOth March, 1905. 



It is -vrell known that some mollusca, as thoy increase in size, 

 withdraw from the upfxir or apical part of their shells. In some cases 

 the empty space is at h^ngth filled with solid matter, as in Ma'jiluH, while 

 in others septa are formed, completely closing the vacant portion, as 

 in EuoinphaluH, Triton, ate. In elongated shells, such as Cylindrdla, 

 Eucalodium, and Rumina decoUata, the vacated portion, becoming dead, 

 breaks off and leaves the shell decollate. 



Kecently, while cutting a shell of Glyptostoma Ne^ir^herryanum to 

 show its internal structure, I notice<l what I at first took to be an 

 empty i>upa-case of some insect, such as are frequently met with in 

 the whorls of shells. Its substance, however, was found to be hard 

 and solid, and a closer examination revealed it to be a saccate septum 



composed of several layers of shelly matter and ha\-ing its convex 

 apex towards the jjrotoconch (Fig. 1). On carefully breaking away 

 some portions of the outer shell-wall higher up the spire, two 

 additional septa were observed, each having been formed successivelv 

 as the animal retreatc*d further from the protoconch. The concave 

 shape of each septum conforms to the shape of the liver-lobe, its apex 

 is obliquely inclined ; and while it closely invests the inner wall of 

 the whorl a narrow s-pace is left between it and the outer wall. The 

 first septum occurs 1^ whorLs from the ap^x of the shell, the next 

 f whorl further down, while another £ whorl divides the second from 

 the third septum (Fig. 2j. A second specimen, upon being opened, 

 yielded similar results. 



As I have failed to trace any mention of such formations in the 

 species under discussion, or in fact in any Helicoid, either in general 

 textbooks or in special American faunal treatises, I have thought it of 

 interest to place the facts on record. 



