138 PROCEEDINGS OF THE NATIONAL MUSEUM Vol.89 



Opisthocoelops, new subgenus 



In this subgenus the operculum and the breathing device are like 

 those of Opisthocoelex. The outer peristome is also broadly expanded, 

 as in Opisthocoelex, but it frequently does not completely close the 

 umbilicus. The axial ribs are gathered into conspicuous hollow tufts 

 at the summit. 



Type: Opisthocoelicum (Opisthocoelops) excurrens ([Gundlachl Pfeif- 

 fer). 



KEY TO THE SPECIES OP THE SUBGENUS OPISTHOCOELOPS 



Posterior angle of aperture strongly auriculated occultum 



Posterior angle of aperture not strongly auriculated excurrens 



OPISTHOCOEUCUM (OPISTHOCOELOPS) OCCULTUM (Torre and Henderson) 



Plate 9, Figure 7 



1920. Opisthosiphon (Opisthosiphona) occuUus Henderson and Bartsch, Proc. 



U. S. Nat. Mas., vol. 58, p. 67, nomen nudum. 



1921. Opisthosiphon (Opisthosiphona) occultum Torre and Henderson, Proc. 



U. S. Nat. Mus., vol. 59, pp. 258-259, pi. 41, figs. 4, 6, 7. 



Shell elongate-conic, flesh colored, sometimes pale brown with 

 interrupted spiral bands of brown; the elements composing these 

 bands are rather distantly spaced and they are arranged in both axial 

 and spiral series ; the peristome is flesh colored, the outer rayed on the 

 outer lip by the spiral bands of brown. Nuclear whorls 1.7, small, 

 well rounded, smooth, except for the last portion of the last turn, 

 which shows the beginning of the postnuclear sculpture, Postnu- 

 clear whorls somewhat inflated, strongly rounded, marked by slender, 

 shghtly wavy, retractively curved axial riblets, which are rather dis- 

 tantly spaced on the first turn and which become gradually more 

 closely approximated as the shell increases in size ; on the last turn the 

 spaces that separate the riblets are narrower than the riblets. Of 

 these ribs, 30 occur upon the first turn, 62 upon the second, 118 upon 

 the third, and 130 upon the last turn of the type. At more or less 

 regular intervals some of these riblets are gathered into rather poorly 

 developed hollow tufts at the summit. The riblets between these 

 tufts are shorter than the rest Suture strongly constricted. Periph- 

 ery well rounded. Base short, strongly rounded, marked by the 

 continuation of the axial ribs, and within the umbilicus by slender 

 spiral cords. Aperture broadly oval; peristome double, the inner 

 slightly exserted and reflected; the outer broadly, somewhat flaringly 

 expanded, deeply notched on the middle of the inner Hp; posterior to 

 the notch the outer peristome is reflected over the umbilicus, which it 

 completely covers, on the parietal wall the outer peristome is adnate 

 to the preceding turn, while at the posterior angle it forms a conspic- 

 uous auricle, which is rendered somewhat irregular by the siphon 

 immediately behind it; on the outer and basal lip it is a trifle narrower 



