194 MONOCEROS. 



M. BREVIDENTATUM, Gray. PI. 60, fig. 294. 



Dark chocolate, encircled by three or four rows of widely 

 separated depressed white nodules; aperture bluish within, 

 margin stained with chocolate. Tooth very small. 



Length, '75-1/25 inches. 



Panama, and W. Coast of Central Arnericu. 



M. GIGANTEUM, Lesson. PL 61, fig. 301. 



Yellowish brown, lighter within. Length, 3-4 inches. 



Chili. 



M. CALCAR, Martyn. PI. 60, figs. 295, 292, 293, 296, 297; PI. 61, 

 figs. 298-300, 302, 303, 307. 



Varying from chocolate-brown, through yellowish to white ; 

 interior and columella usually white, with frequently a chocolate 

 border within the margin of the lip. Surface in the typical form 

 covered with closely imbricated revolving riblets and the lip but 

 slightly thickened within the margin. Length, 2-2*5 inches. 



Cape Horn to Chili. 



A species which, like Trophon Geversianus from the same 

 region, exhibits much variation. At Cape Horn the typical 

 form, as described above, occurs, and this, when the spire is 

 more exserted, has received from Lamarck the name of imbri- 

 catum (fig. 296). The surface varies to nearly smooth with 

 simply revolving lines occasionally, becoming M. striatum of 

 Lamarck (fig. 302). 



On the Chilian coast the species is usually heavier, the whorls 

 very thick, smooth or with revolving striae or riblets, the outer 

 lip very much thickened within and obscurely dentate. This 

 form may, for convenience, be designated as var. crassilabrum, 

 Lam. (fig. 307), although its union with the type is apparent 

 from long series of transition forms. With this must be united 

 M. glabratum, Lam. (fig. 300), and M. globulw, Sowb. (fig. 298), 

 as well as M. costatum, Sowb. (figs. 292, 297). M. citrinum, 

 Sowb., is a mere color variation, and M. acuminatum of the same 

 author (fig. 303) is a form with more exserted spire. 



