76 PROCEEDINGS OF THE NATIONAL MUSEUM vol. lo© 



from 120 to 741 feet. The stations at 217, 341, 420, and 453 feet 

 jnelded 14, 31, 17, and 26, respectively. About 10 of the specimens 

 from 341 feet range in size from 7.7 mm. high by 8.6 mm. in diameter 

 to 10.2 by 11.8 mm.; the others range from the latter size up to 

 20 mm. in height. Two specimens from 477 feet measure 5.7 mm. high 

 by 6.5 mm. in diameter and 10 mm. by 10.3 mm. Two from 453 feet 

 are 20.4 by 19.7 mm. and 17.9 by 18.3 mm.; one from 184 feet is 



20.4 by 18.8 mm., and one from 120 feet (Aug. 8, 1949) is 20.8 by 



18.5 mm. The younger specimens are broader than high and the 

 larger ones are higher than broad. 



Other material examined : Varieties of M. costalis from the mouth 

 of the Mackenzie River, Peard Bay and vicinity, and "north of 

 Bering Strait"; numerous specimens from Melville Island, Green- 

 land, Labrador and Newfoundland listed under various names; and 

 specimens of typical M, costalis from south of Bering Strait and from 

 Greenland and Norway. 



Discussion : Typical specimens of M. costalis are characterized by 4 

 to 6 prominent spiral ridges or cords that lend an angular appearance 

 to the whorls, they are angled at the periphery of the last whorl 

 and the base is flattened; the axial folds or ribs of the apical whorls 

 give the latter a nodulus appearance, and the spiral ridges are alter- 

 nated with secondary threads or lirae. The Point Barrow specimens 

 are not typical M. costalis; they belong rather to the variety gravdis 

 Morch (pi. 1, fig. 1) and the larger forma mvltilirata Odhner (pi. 1, 

 figs. 5, 7). In both of these forms the spiral ridges are more numerous, 

 from 5 to 8; they are often uniformly spaced without alternating 

 smaller spiral lirae and, when present, the alternating lirae are not very 

 pronounced and number only from 1 to 4, perhaps 5, on the last whorl. 

 The shell is not angled at the periphery of the last whorl and the base 

 is rounded. In var. grandis the base may be without any spiral lirae, 

 or there may be from 1 to 6, with perhaps 2 to 3 around the strong cord 

 outlining the umbilicus and 2 or 3 more near the margin (pi. 1, fig. 2). 

 In forma multilirata, the base may have from 6 to 12 spiral lirae or 

 cords (pi. 1, fig. 7), all of which may be uniformly strong, or uniformly 

 weaiv, or may vary in strength in the same shell. The number of basal 

 lirae seems to have little or no relation to the size of the shell. In 

 both var. grandis and forma multilirata the nodulous appearance of the 

 apical whorls is much less pronounced than in typical AI. costalis. 

 In some specimens from Melville Island the cord outlining the umbili- 

 cus is very weak, and in some the incremental lines are crowded into 

 raised groups at intervals, detracting from the silky appearance charac- 

 teristic of shells of this species. In the Point Barrow specimens and 

 in the other specimens examined from other northern localities, there 



