A TAX0N0MIC STUDY ON PYROSOMA METCALF AND HOPKINS. 229 



All the known specimens of P. verticillatum cylindricwm are from 

 the northern Pacific Ocean in the vicinity of the Philippine Islands, 

 and were taken by the steamer Albatross during the Philippine Expe- 

 dition of 1907-1910 at the following stations: 



D. 5120, Sombrero Island, Verde Island Passage; Jan. 20, 1908; 393 

 fathoms; surface temperature, 43.7° F. ; surface density, 1.02386; 

 one specimen, Cat. No. 6414 U.S.N.M. 



D. 5125, Nogas Island, Sulu Sea, vicinity southern Panay; Feb. 

 3, 1908; 411 fathoms; surface temperature, 80° F. ; surface density, 

 1.02444; seven specimens, Cat, Nos. 6468 (Holotype) and 6412 

 (Paratype) U.S.N.M. 



D. 5320, China Sea, vicinity of Formosa, Nov. 5, 1908; 1,804 

 fathoms; surface temperature, 80° F.; one specimen. 



D. 5437, Hermana Mayor Light, west coast of Luzon; Apr. 9, 1909; 

 100-600 fathoms; surface temperature 86° F.; two specimens. 



D. 5456, Utara Point, Bongo Island.; May 22, 1908; 158 fathoms; 

 surface temperature, 86° F.; surface density, 1.02262; ten specimens, 

 Cat, No. 5456 U.S.N.M. 



PYROSOMA HYBRIDUM, new species. 



Plate 36, figs. 54 and 55; plate 23 ; figs. 16 and 17. 



This is a fairly distinct form, representing to some extent a condi- 

 tion intermediate between the preceding species and P. dliplicum. 

 It is not likely that it is a hybrid form, for there have been collected 

 a considerable number of specimens, 16 in all, from four localities. 

 Moreover, there are certain characters peculiar to this species which 

 serve to distinguish it from all nearly related species. It seems not 

 unlikely, however, that such varieties as the present one may have 

 arisen originally as the outcome of hybridization. Its nearest rela- 

 tive is P. verticillatum,, but it seems a little too divergent to be properly 

 classed as a variety of this species, especially as no intergrading forms 

 are known. 



The colony is laterally flattened in all cases, and appears oval when 

 viewed from the side (fig. 54). The relative dimensions are some- 

 what variable: An average specimen is 2.4 cm. long, 1.7 cm. wide, and 

 0.4 cm. thick. Larger specimens — the largest is 4.2 cm. long — are 

 relatively more narrow. The ratio of the width to the thickness of 

 the colony is about 4 to 1. All the zooids, and to some extent the 

 test, are darkly stained bluish-gray, never yellow; the testes when 

 mature appear as black bodies. A vcrticillate arrangement of zooids 

 in transverse whorls obtains here, as in the preceding forms. In all 

 but the older specimens they display a progression in size from one 

 end to the other of I lie colony, those near the. diaphragm being small 

 and closely packed together, and those at the closed end largest. 



