60 JAPANESE MOLLUSKS, VERMETID^, 



such planorboid forms as T. masier Dh., T. atra Rouss, etc. This 

 stage is shown in the cluster pictured on pi. IV. The subsequent 

 growth is erect and but slightly spiral, the tube generally contracting 

 slightly in diameter, and cylindrical in form. The sculpture consists 

 of narrow longitudinal cords at rather wide intervals, the spaces 

 being occupied by a variable number of threads, (usually three) of 

 which the middle one is larger. Irregular growth-lines crenulate 

 the longitudinals. This sculpture is normally developed upon all 

 sides of the tube. Upon the latter part of the free portion it is often 

 subobsolete. The aperture is circular in adult shells, at right angles 

 to the tube, and white within. The external surface is of a pale 

 brown tint. No internal septa were found in numerous broken spec- 

 imens; and although the shells were apparently alive when collected, 

 no opercula were preserved in them, the absence of this organ being 

 one of the characteristics of Thylacodes. 



The dimensions may be seen in the figures, which are of natural 

 size. The diameter of the tubes at the aperture averages 13 mm. 



The embryonic shell, at the time it becomes attached, is bulimiform, 

 glossy and smooth ; lying upon its side. The first whorl formed 

 after attachment takes place is perfectly planorboid, showing the 

 nuclear shell in the center. 



This species differs from Vermetus imhricatm Dkr. in lacking the 

 imbricating growth-strije of that form. As figured by Dunker V. 

 imbricatus is a planorboid form, like V. masier Dh., atra Rouss, etc., 

 but it may possibly be an immature shell. 



T. medusce is apparently allied to V. polyphragmus Sassi, V. denti- 

 jerus Lam. and V. novce-hollandioe Rouss, but it differs from them 

 in having the sculpture developed over the whole circumference of 

 the tube, not confined to its upper surface, or the part corresponding 

 to the base of the shell in ordinary gasteropods, as it is in those spe- 

 cies. 



Spiroglyphus planorbis Dkr. Moll. Jap., p. IS, pi. 2, f. Ifi 

 Deshima, on T. imhricatus. (Dkr.). 



Siliquaria (Pyxipoma) lactea Linne. Man. Conch., VIII, p. 191, pi. 58, f. 2G. 

 Japan (Dkr.). 



Siliquaria (Agathirses) cumingii Morch. Man. Conch., VIII, p. H)0, pi. 57, f. lit; 

 pi. 58, f. 21. 



Nemoto, Boshiu (Stearns) ; Bay of Yedo (Lischke). 



