THK NAUTILUS. 



139 



Length 36.5, diam. 8.7 mm.; whorls remaining 



Length 32, diam. 8.2 mm.; whorls remaining 10^. 



At the Mecos Falls locality there is a small, slender form occur- 

 ring with the types, having only very slight, hardly noticeable nodes 

 on the spiral swelling of the axis. The aperture is carried forward 

 further than in the typical examples. Two of these measure: 



Length 35, diam. 7.1 mm.; whorls remaining 12^. 



Length 32.2, diam 7.1 mm.; whorls remaining 12. 



C^LOCENTRUM ISCHNOSTELE, n. Sp. 



The shell is shortly rimate, imperforate, the lower half cylindric, 

 upper half slowly tapering to the truncate summit; thin; pale 

 brown. The surface has a silky luster, and is densely sculptured 

 with thread-like ribs, which arch backward, and are about as wide as 

 their intervals except on the last whorl, where they are finer and 

 more closely crowded. On the penultimate whorl there are about 

 90 (88 to 92) ribs. The breach at the summit is closed by a very 

 convex, granulose plug. Remaining whorls ll to 14, slightly 

 convex, having a minute carina below the suture and very close to 

 it. The last whorl is convex and has a scarcely noticeable basal 

 cord ; its last half is straightened, and projects shortly (about 1 

 mm.) forward. The aperture is oblique, rounded-ovate, the upper 

 margin straightened. Peristome obtuse, narrowly recurved through- 

 out. The internal axis is very slender throughout, weakly sinuous 

 within each whorl, smooth, its diameter contained 8^ times in that of 

 the shell. It opens by a minute perforation at the summit. 



Length 32, diam. 6.8 mm.; 13 whorls remaining. 



Length 32.8, diam. 6.2 mm.; 14^ whorls remaining. 



Length 30, diam. 6.5 mm.; ll whorls remaining. 



Length 27.8, diam. 6.4 mm.; 12 whorls remaining. 



Length 27.8, diam. 6.2 mm.; ll whorls remaining. 



This form occurred in the same locality with Streptostyla bartschi, 

 near Mecos Falls. It stands close to the preceding species, but 

 differs by its much more numerous ribs, the much more slender 

 internal axis, and the last whorl does not run forward so far. It 

 is remarkable for the small size of the internal column. 



In a young shell 11 mm. long with 14 whorls the first 1^ whorls 

 forming the hemispherical summit are perfectly smooth and measure 

 1^ mm. in diameter. Then very fine very short riblets appear 



