314 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



the mouth shields broader, shorter, and almost equally broad toward 

 both extremities, distal and proximal, by having the first under arm 

 plates broader than long, and by never having the dorsal arm spine 

 thickened or claviform. 



OpMocoma canaliculata is only known from a single specimen 

 which I redescribed in some detail and figured in 1904 ('04a, p. 75). 

 It is the only species of OpMocoma in which the adoral plates are 

 developed in front of the mouth shields, and are broadly united to 

 each other in the median interradial line. Lyman ('82, p. 168) says 

 that 0. canaliculata possesses small imbricated scales at the base of 

 the first dorsal arm spine, a feature which Liitken did not mention; 

 neither did I mention it in my description in 1904, and I do not find 

 in my notes any mention of it ; moreover, I do not find in the photo- 

 graphs which I took of the unique type preserved in the Copenhagen 

 Museum the least trace of it. However that may be, O canaliculata 

 stands quite by itself in the genus OpMocoma and perhaps the study 

 of other specimens will one day show that it must be placed in a 

 new genus. 



OpMocoma bollonsi from New Zealand appears to be very close 

 to O. scolopendrina, but it possesses six arm spines at the base of 

 the arms. 



As for OpMocoma lubrica, this species has two tentacle scales 

 throughout the whole length of the arms, and the adoral plates are 

 confined to the sides of the mouth shields; it is a true OpMocoma, 

 which is especially characterized by the peculiar form of the under 

 arm plates. 



OpMocoma insularia from the Hawaiian Islands was described by 

 Lyman as a distinct species in 1861, but it was later united by him 

 with O. brevipes. H. L. Clark again considered it as a distinct 

 species, and he gave two photographs of it in 1915 ('15, pi. 15, figs. 

 3,4). 



It may be noticed that I have not recorded above among the At- 

 lantic species of the genus OpMocoma O. nigra from the North 

 Atlantic. This species, which runs down from the coast of Norway 

 along the coast of the British Isles and of France as far as the Azores 

 and enters the Mediterranean sea, it seems to me must be removed 

 from the genus OpMocoma to form an independent genus. OpMo- 

 coma nigra is distinguished from all the known species of the genus 

 OpMocoma by its hollow arm spines and it was for this reason that 

 Lyman assigned it to the genus OpMoconis, comparing it with OpMo- 

 conis miliaria and O. antarctica. Furthermore, OpMocoma nigra 

 shows broad and thick peristomial plates very different from those 

 which are known in the other species of OpMocoma; I include a 

 photograph of an internal view of the mouth pieces of O. nigra (pi. 



