VOL, 2, PT. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 139 



Under date of March 10, Professor Ritter writes me: 



Curiously enough, although the aggregated form of this species is one of the most 

 abundant in this region, I have seen only two specimens of the solitary form. 



Iii a later letter, referring to his paper on the Pelagic Tunicata cf 

 the San Diego region, he writes: 



At the time I had seen but one specimen of the form. Since then the number has 

 been increased to two by the handling of thousands of specimens. 



It will be seen, therefore, how abundant the aggregated form of 

 this species is, and how rare are fidl grown adults in collections thus 

 far made. What may be the explanation of this condition ? Thalia 

 democratica, the most abundant and the most widely distributed of the 

 Salpidae, shows conditions of vertical distribution that are suggestive 

 in this connection. The solitary individuals lie at considerable 

 depth during winter, spring, and early summer, coming to the surface 

 with the aggregated zooids in the fall. It seems not unlikely that 

 the solitar} r Pegea confederata lies generally in deep water the year 

 round, a few wholly adult individuals coining to the surface only 

 occasionally, the great majority probably never coming to the surface 

 except more or less by accident. The Plankton Expedition reports 

 51 solitary individuals (including embryos) collected in four hauls of the 

 vertical net. These may have been lying at any depth. Ir the Siid- 

 polar Expedition collections, three solitary individuals are reported from 

 the surface, nine from a depth of 10 meters, three from a depth of 

 20 meters, and three from an unknown depth. How many of these 

 were embryos was not stated. The Tiefsee Expedition reports seven 

 of the solitary form from the surface and five from unknown depths. 

 In this case again it is not stated whether the individuals were fully 

 formed adults or embryos. More data as to the vertical distribu- 

 tion of Pegea are much to be desired. Such data as we have support 

 the suggestion that the solitary individuals seldom reach the surface, 

 but may be much more abundant at some depth. 



PEGEA CONFEDERATA, subspecies BICAUDATA (Quoy and Gaimard, 1826). 



S. nephodea Lesson, 1830. 



Salpa bicaudala Quoy and Gaimard, 1826. 



The solitary form of this salp has not been clearly distinguished. 

 In fact, most recent authors deny to bicaudata any consideration even 

 as a distinct variety. The aggregated zooids of the Pegea which 

 Streiff studied and described as P. confederata belonged to the form 

 bicaudata, and the presumption is that the solitary individuals were 

 of the same subspecies to which the aggregated zooids belonged, 

 but of course this is not certain. His specimens of the solitary form 

 were very different from mine, in the character of their atrial muscu- 

 lature, and were somewhat different in the character of the oral 



