114 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Mantle thin, not adherent to the test in preserved specimens ex- 

 cept in the region of the apertures. Musculature very slight or 

 almost wanting on most parts of the body, but slender bands radiate 

 from the apertures and generally unite in groups to form larger 

 bands. These, however, do not extend far from the apertures except 

 those near the two angles of the mouth-like cleft. There the bands 

 are long and stout and closely grouped, and serve to move the valve 

 or cover. 



Tentacles simple, rather small and widely spaced, about 32 in 

 number of 3 orders rather regularly arranged; in addition there 

 are a few fourth-order tentacles in some parts of the circle. (This 

 count was made in one of the largest specimens.) 



Dorsal tubercle C-shaped with strongly incurved horns; the open 

 interval directed forward. 



Figs. 68-71. — Rhgdosoma papillosum (Stimpson). 68, External view. X 1.25. GO, 

 Dossal tubercle, part of dorsal lamina, and part of the circle of tentacles. 

 X 10. 70, Part of branchial sac. X 15. 71, Stomach, part of intestine, and 



GONADS AS SEEN THROUGH MANTLE. X 2.5. 



Dorsal lamina represented by a series of small, rather slender lan- 

 guets. 



Branchial sac without folds or minute plications. Transverse 

 vessels very numerous, of two sizes placed alternately in most parts 

 of the sac, but in some places nearly uniform in size. The intervals 

 between the transverse vessels are divided by very numerous slender 

 longitudinal vessels into rather narrow stigmata. At intervals of 

 3 or 4 stigmata small curved papillae arise from the transverse 

 vessels and support slender internal longitudinal vessels. They pro- 

 ject a trifle^ beyond their point of union with the internal longi- 

 tudinal vessels. The latter in large and old individuals are mostly 

 incomplete and interrupted between the supporting papillae so that 

 they form merely lateral branches of these papillae. (See fig. 70.) 

 In one of the smaller and doubtless much younger individuals the 

 internal longitudinal vessels were found to be much less reduced and 

 were practically complete over considerable areas in some parts of 

 the sac. Here and there one of the transverse vessels tapers off 



