A TAXONOMIC STUDY ON PYKOSOMA — METCALF AND HOPKINS. 239 



resemble closely the adult of the typical P. atlanticum atla/nticum, 

 diverging more and more from this central type as they mature. 



PYROSOMA ATLANTICUM ATLANTICUM. 



Plates 26 and 27; plate 28, fig. 33; plate 34, figs. 45 and 46; plate 36, fig. 53. 



If we examine specimens of P. atlanticum atlanticvm 1 from the 

 Pacific Ocean, where the form seems to be most characteristically 

 represented, we find remarkable variation among them in regard to 

 size, color, and organization. The great range in size displayed here 

 is in itself significant, when we contrast this with the species verti- 

 cillatum, hybridum, oyerculatum, and elli/pticum described previously, 

 which are all small in comparison. The majority of specimens of 

 P. atlanticum atlanticum in the Albatross Pacific collections do not 

 exceed 8 cm. in length; the largest is only 12 cm. long. 



Bitter (1905) has described as P. giganteum a type of Pyrosoma, 

 found on the California coast, which agrees in almost all particulars 

 except size with the one we here describe as P. atlanticum atlanticum. 

 According to our distinction between these two forms, it would seem 

 clear that he was dealing with material representing the latter. He 

 was himself very uncertain about assigning his specimens to either 

 one of the two species, as they were then treated. If his specimens 

 were, as it seems, true atlanticum, it may be stated that this form 

 is known to attain a length of 60 cm., for Ritter's largest specimen 

 was of this size. He reports also several other exceptional specimens 

 whose measurements are given as follows (Ritter, 1905): "Measure- 

 ments of three largest preserved colonies: first, length, 25 cm., great- 

 est thickness near open end 2.5 cm.; second, length, 25 cm., greatest 

 thickness, 3.5 cm.; third, length, 19 cm., greatest thickness about 

 middle of length, 3 cm.; largest colonies observed, measured in life, 

 60 cm. long, 40 cm. long, and 35 cm. long." 



The form of the colony in the majority of specimens is conic- 

 cylindrical, the open end of the colony being wider than the closed. 

 There are numerous departures from this shape; some individuals 

 are less conical, more truly cylindrical, than others; some are thickest 

 in the middle and taper toward both ends; while in others there is 

 the suggestion of natural flattening at the open end of the colony. 

 That this flattening is not entirely due to artificial causes (shrinking 

 or compression from poor preservation) is evident, for, if the diaphragm 

 at the opening of the colony is examined, it will be found to be adapted 

 to the form of the slit-like opening, as was shown for P. ellipticum 

 (fig. 20, pi. 23). On the other hand, some quite mature colonies 

 show no such flattening. From the data quoted above from Ritter 

 it will be seen too that the ratio between length and thickness differs 



i According to taxonomic conventions there seems no escape from the absurdity of naming these Pacific 

 forms atlanticum. 



