128 SYNAPTIDAE. 



Only twenty-three wheels were suitable for counting the marginal teeth and in 

 these there is great diversity, the number ranging from twenty-one to thirty; 

 the average number per wheel is a trifle more than twenty-four, or just about 

 twice the number of spokes; the largest relative number is twenty-seven teeth 

 for ten spokes and the smallest is twenty-two teeth for thirteen spokes. Hub 

 of wheel small and solid, its diameter .20-.25 of wheel diameter. Wheels range 

 from 190 to 315 ^ across with the marginal teeth 30-50 n long. 



Station 4647. Eastern Tropical Pacific, 4° 33' S., 87° 42' 30" W., 2,005 fma. Bott. temp. 35.5°. Lt. gy. 

 and br. glob. oz. 



Two specimens. 



The occurrence of two species of Myriotrochus south of the equator and 

 at a depth of over two thousand fathoms is one of the most extraordinary zoo- 

 logical discoveries made by the Albatross during her 1904-1905 cruise. The 

 genus has not hitherto been known south of Norway, Newfoundland, Alaska, 

 and northern Japan, nor at a depth of more than a few hundred fathoms. The 

 species taken at station 4647 are very different in general appearance from any 

 of the known forms, but giganteus is much like the common subarctic species 

 rinkii in the character of its wheels. The wheels of giganteus average a trifle 

 larger than in rinkii but the difference is insignificant. The spokes, however, 

 are noticeably fewer (10-14, average almost twelve, as against 12-22, average 

 seventeen) and much wider, while the hubs are larger (.20-25 wheel-diameter 

 as against about .16). The marginal teeth are relatively more numerous in 

 giganteus, where they average almost exactly double the number of spokes. 

 The wheels in bathybius are conspicuously different from those of any other 

 member of the genus. Both the tropical species seem to be much larger and 

 stouter than rinkii, the largest of the previously known species, although there 

 is not great difference in length, since Ostergren reports specimens of rinkii 

 up to 70 mm. long in life. The superficial resemblance of giganteus in form and 

 color to a Molpadia is very striking and only the examination of the calcareous 

 particles proves how misleading the resemblance is. 



It was hard to convince myself that the holotype of bathybius was not 

 merely an aberrant individual of giganteus, but the wheels are too markedly and 

 consistently different to support such an idea. Moreover the differences in the 

 digitation of the tentacles and in the calcareous ring cannot be ignored and no 

 other conclusion seems possible than that these two closely-allied species live 

 together on the ooze in the great depths of the eastern Pacific. It is an 

 environment peculiarly suited to holothurians, as is well shown by the fact that 

 nine different species were taken in the single haul of the trawl at station 4647. 



