294 bulletin: museum of comparative zoology. 



dusky purplish coloring on the upper arm-plates and that only near 

 the tip of the arm. 



This species is well-characterized by the disk-covering, the single 

 tentacle-scale, and the oral papillae. In the first character it resembles 

 the Japanese Amphiura trachydisca but in no other particular is it 

 like that species. It is certainly a noteworthy fact that this species 

 was dug from among "eel-grass" (or "turtle-grass") roots on a bot- 

 tom of firm sandy mud close to mangroves, in company with Ain- 

 phioplus abditus and A. coniortodes. No other brittle-stars were 

 found there and of these three species, only A. abditus was found at 

 the Tortugas during the following weeks. 



Ophiactis. 



This is one of the large genera of brittle-stars to which species have 

 been added year after year without any adequate comparison of the 

 different forms with each other. The genus was established by 

 Lutken in 1856, who included at the time only two species, 0. krebsii 

 and 0. mulleri, though he added four others a month later. He also 

 suggested that 0. ballii might be properly placed in the genus. Liit- 

 ken's diagnosis of the genus is brief but sufficiently explicit, while the 

 general facies of the species is usually quite characteristic. Indeed 

 too much reliance has been placed on the general facies and as a 

 result a number of species have been referred to the genus erroneously. 

 Lutken and Mortensen, (1899. Mem. M. C. Z., 23, p. 143) called 

 attention to the fact that certain amphiurans, lacking a pair of sub- 

 dental oral papillae, had been wrongly placed in Amphiura, and on 

 the strength of this one character transferred them to Ophiactis. 

 Recent writers have followed this arrangement but Matsumoto, (1915. 

 Proc. Acad. nat. sci. Phila., 67, p. 67) called attention to the fact that 

 Ophiactis had thus become a very unnatural group, and he removed 

 several species to a new genus Amphiactis, which he placed in a sepa- 

 rate family. When the Catalogue of Recent Ophiurans (1915. Mem. 

 M. C. Z., 25) was published I was not convinced of the necessity of 

 this action but after critical study I am satisfied Matsumoto is 

 right. 



As restricted Ophiactis includes those amphiurids with genital slits, 

 no paired subdental papillae, 1 or 2 oral papillae on each side of the 

 jaw (in rare individual cases, 3 may be present on some of the jaws) 

 and a single large tentacle-scale. The last, although not mentioned 



