154 HAWAIIAN AND OTHER PACIFIC ECHINI. 



graphical separation from that species. The size and arrangement of the 

 abactinal pores are so different from what is found in specimens of placenta 

 of the same size as the type of rigidum, that we think it better to keep the 

 latter separate, at least until moi'e material is available. Doderlein con- 

 siders a form of placenta which he calls Sigsbei recognizably distinct from 

 placenta proper, because of fewer and higher abactinal coronal plates. 

 Mortensen (1907) finds that this difference is not constant, but believing 

 the tridentate pedicellaria? of true placenta to differ from those of " Sigs- 

 bei" he thinks the latter may be a recognizable variety. Examination 

 of a considerable series of specimens of both P. placenta and P. bursa- 

 rium has satisfied us that in those two species, the height, and conse- 

 quently the number, of coronal plates is subject to considerable individual 

 diversity not associated with definite localities, and that the tridentate 

 pedicellarioe are also very variable. Specimens from the same or adjoining 

 stations have very different pedicellarias; the best illustration of this is 

 found in two specimens of bursarium from Stations 5082 and 50S4 ; the 

 specimen from 5082 has only short and thick tridentate pedicellariaB, while 

 in the one from 5084 these pedicellaria; are longer and more slender than 

 in any Phormosoma we have seen. It may be well to add that other speci- 

 mens are intermediate between these two extremes. In view of these facts, 

 we do not think that the form to which Doderlein and Mortensen refer can 

 well be distinguished, but we are now satisfied that P. Sigsbei is really as 

 valid a species as most of those in the genus, if we recognize others than 

 placenta. The name was originally applied to the Phormosoma collected by 

 the " Blake " west of the Lesser Antilles. Subsequently this form was con- 

 sidered identical with the Phormosoma collected off the eastern coast of the 

 United States, and all were determined as P. placenta. Recent examination 

 of the Caribbean specimens, and comparison with large numbers of Phormo- 

 soma from both the Atlantic and Pacific Oceans, have led us to believe that 

 they are distinct from placenta and somewhat nearer bursarium. We have 

 decided therefore to revive the name Sigsbei for them ; if the American 

 form of placenta, usually having short, thick pedicellariaa and relatively few 

 coronal plates is to be distinguished from the European form, a new name 

 must be given to it. In this connection it may be well to state that a 

 renewed examination of the specimens of young Phormosomas, the figures 

 of which ("Blake" Echini, PI. XV) are criticised by Mortensen ("Ingolf" 

 Ech., Pt. I, p. 69), has satisfied us that those figures are essentially correct. 



