50 BULlrETESr 181, UNITED STATES NATIONAL MUSEUM 



which are rather distantly spaced on all the whorls. On the later 

 turns axial hairlines are also apparent in the intercostal spaces. 

 These are irregularly distributed and of irregular numbers. Suture 

 very strongly constricted. Periphery low, well rounded. Base de- 

 cidedly produced, flatly umbilicated with a decidely elevated keel at 

 the outer limit of the umbilicus, which is rendered crenulated by the 

 axial riblets that extend also over the umbilical wall. Aperture sub- 

 circular; peristome sinuous, that is not in a flat plane, moderately 

 thickened and moderately excised on the parietal wall. Operculum 

 typically megalomastomid. 



The specimen described and figured, U.S.N.M. No. 535943, comes 

 from St. Thomas. It has 8.2 whorls and measures : Height, 15.4 mm. ; 

 greater diameter, 6.2 mm. 



There are 17 additional lots of this species, all from St. Thomas, in 

 the collection of the United States National Museum. 



This species is nearest related to M. {M.) antilJanim (Sowerby), 

 from which its more elongate form, more distantly spaced ribs, and 

 more pronounced umbilical keel will readily differentiate it. 



MEGALOMASTOMA (MEGALOMASTOMA) PETITI, new species 



Plate 9, Figures 16-18. 20 



Shell elongate-ovate, sooty chestnut-brown; peristome white. 

 Nuclear whorls 1.3, well rounded, smooth. The early postnuclear 

 whorls somewhat inflated, strongly rounded, the last one slightly flat- 

 tened and marked by slender sublamellar, rather distantly spaced axial 

 riblets, which are rather irregularly distributed. Suture well con- 

 stricted. Periphery well rounded. Base rather produced, flatly um- 

 bilicated, with a moderately strong cord at the outer limit of the 

 umbilicus. Aperture circular; peristome almost flat, moderately 

 thickened, slightly reflected and moderately excised on the parietal 

 wall. Operculum typically megalomastomid. 



This species comes from the Island of St. John, The type, U.S.N.M. 

 No. 535942, was collected by me on Bordeaux Hill, Hurricane Harbor, 

 St. John. It has 7 whorls and measures : Height, 12.3 mm. ; greater 

 diameter, 5.4 mm. 



The United States National Museum contains six additional lots, all 

 from St. John. 



This species is easily distinguished from the other Megalomastomas 

 by its shorter and much stouter shape. 



Megalomastomoides, new subgenus 



Shell resembling Megalomastoma in size, shape, and periostracal 

 covering but differing from it in having the peristome not excised on 



