108 



ovato-conica, atropurpurea, liris granulosis ubique cincta : spira 

 anfr. 5-6, ultimo subangulato : basis convexiuscula, late per- 

 forata; umbilico cavernoso, limine acuto, denticulato, prope colu- 

 mellam interrupto: apertura circularis ; columella obliqua, acuta, 

 antice dentata, emarginata ; labro albo et nigro articulato, intus 

 sulcato ; fauce margaritaceo. Alt. j^^, lat. y^g- poll. Habitat, 

 Tutuilla, Navigator Islands. 



Allied to T. pharaonis and T. canaliculata, in its habitus and 

 umbilicus, but more like T. corallinus in size and general charac- 

 ters. Its uniform color and granulation, toothed margin of the 

 umbilic, with the conspicuous tooth above, are its characters. 



Trochus (Monodonta) rotellinus. Testa parva, solida, len- 

 tic^ilaris, nitida, fissura latebrosa perforata, dilute incarnata, 

 flammulis flexuosis saturatioribus, et seriebus duobus transversis 

 macularum fuscarum variegata : spira anfr. 5, convexiusculis : 

 apertura parva, subcircularis ; columella perobliqua, arcuata pos- 

 tice ab anfractu sejuncta, antice in dentem acutum terminante ; 

 labro prope dentem quasi emarginato. Diam. f, alt. ^ poll. 

 Hah. Mangsi Island. 



This curious little shell would at first be taken by its form, 

 polish, and color, for a Rotella, while the notch of the aperture 

 resembles that of Buccinum neriteum. But its umbilic and col- 

 umella place it with Monodonta, or more properly in that sub- 

 division of Trochus named Clanculus by Montfort. 



Mr. Desor addressed the Society upon the subject of the 

 distribution of animal life among the Shoals of Nantucket. 

 At a former meeting he had given descriptions of the various 

 species found in this locality, and he now proposed to con- 

 sider the law of their distribution. 



The Shoals off Sancati Head, he said, might be regarded as 

 a vast sub-marine plateau, with a depth of water upon it, at no 

 place greater than twenty-five fathoms. Its surface rises into 

 four principal ridges, which approach the surface of the water at 

 different places to within fifteen, ten, six feet, or even one foot. 

 The varying depth of water between these ridges gives rise to 

 four principal horizontal divisions, marked by the absence or 

 the distinct characters of animal forms. 



