PH011M0S0MA ALTERNANS. 155 



Mortensen dismisses the possibility of the differences between his specimens 

 and these figures being specific, by saying he has had specimens from the 

 Gulf of Mexico which are exactly like those from Davis Strait, where his 

 young specimens were taken. This is interesting as confirming our opinion 

 that the Lesser Antillean species is distinct, and also as showing that 

 placenta extends its range as far southwestward as the Gulf of Mexico. 



The eight species of Phormosoma which we here recognize may be dis- 

 tinguished from each other as follows : 



'S" 



Primary tubercles of ambulacra, abactinally, very much smaller than 



those of interambulacra and arranged in four vertical series .... altemans. 

 Primary tubercles of ambulacra, abactinally, not conspicuously smaller 

 than those of interambulacra. 

 Abactinal primary spines rough with minute teeth which are in 



whorls on distal part of spine verticillatum. 



Abactinal primary spines smooth, or at least never verticillate. 



Abactinal primary tubercles small and rather numerous (300- 

 500), occurring on all or nearly all the coronal plates. 

 Ambulacral pores large, in a nearly vertical series at 



extreme outer end of ambulacral plates, abactinally . . rigidum. 

 Ambulacral pores smaller, in more oblique arcs of three 

 abactinally, and not at extreme outer end of plates. 

 Actinostome small (less than .30 h. d.), little larger 



than abactinal system adenicum. 



Actinostome more than .30 h. d., distinctly larger 



than abactinal system placenta. 



Abactinal primary tubercles larger and less numerous (70- 

 300), wanting on many of the upper coronal plates. 

 Abactinal arcs of pores few and nearly vertical, rarely 

 exceeding ten even when there are nine interambulacral 



plates Sigsbei. 



Abactinal arcs of pores 12-25, quite oblique. 



Abactinal primary tubercles few (70-150), large, con- 

 fined to peripheral half of test indicum. 



Abactinal primary tubercles more numerous (150- 

 300), smaller, not confined to peripheral half of 

 test hursarium. 



Phormosoma alternans de Meij. 



Phormosoma alternans de Meijere, 1903. Tijdschr. Ned. Dierk. Ver., (2) VIIT, p. 2. 

 1904. Ech. " Siboga " Exp., Pis. Ill, figs. 21, 22 ; XII, figs. 148-14$. 

 Dutch East Indies ; 386 fathoms. 



Although this species is known from only a single specimen (very dark 

 brown, 52 mm. in diameter), it seems to be better characterized than any of 

 the other species recognized as distinct from placenta. Its general facies, 

 both with and without the spines, would seem to be quite different from that 



