189I.] BAKER — SHELLS FROM THE MAURITIUS. 39 



Magilus striaius, Riippell, Trans. Zool. Soc, i, 259, t. 35 



f. 9, 10. 



Magilus tenuis, Chenu. 111. Conch., t. i, f. S a. 



I have before me seventy specimens of this species in every 

 degree of growth from young to adult, so that I am able to place in 

 the synonomy several species which, from the published figures, seem 

 quite distinct. The Af. costatus of Sowerby I am able to connect with 

 antiquus through a very extensive set of specimens which shows that 

 the ribs, on which the species is founded, are but pronounced exam- 

 ples of the spiral stride so characteristic of antiquus. 



One very young form before me is smooth, and very much like 

 Rapa papyracea in form. The young shells show a great variety in 

 sculpture, from almost smooth, to a wonderful degree of scabrosity. 

 As the shell gets older and prepares to settle down, it thickens the 

 aperture, and the lines of growth become thicker and more crowded 

 together ; as it forms its tube it widens the aperture and starts a keel 

 from the base of the columella, which is persistent throughout the life 

 of the animal. The tubes are often twisted into many different shapes, 

 some twisting in a cork-screw-like manner and others simply bending 

 or curving ; still others are nearly straight. 



Many of the larger tubes are quite heavy and one now before me 

 must weigh nearly a pound, and measures ten inches in length, 

 although somewhat curved, and would measure a foot if straightened 

 out. 



One of the specimens fortunately contained the operculum ; this is 

 oval, concentrically laminated, twelve mill, in length and six in width ; 

 the nucleus is lateral. It is much too small for the aperture of the 

 tube, and would seem to be of but little use to the animal. 



M. Riippellii, (pi. 8, f. 5) Desh., may be retained as a variety 

 characterized by a narrower shell and longer aperture. 



Genus LEPTOCONCHUS, Riippell. 



In this genus the adult does not (is not known to) form a tube 

 and there is no operculum ; otherwise the animal resembles that of 

 Magilus. 



Leptoconchus Cumingii, Desh., PI. 9, f. 4, 6. 

 Conch. He Reunion, 125, t. 12, f. 340, 1863. 



Magilus antiquus, Sowb., Conch. Icon., f. i b. 



Magilus g/obulosus,'Dtsh.,?)0\\h. in Conch. Icon., sp. 10, 1872. 



