ABYLOPSIS TETRAGONA. 225 



nym, no matter how incorrect the description and figure on which the older name 

 rests. That this course must be followed, to accord with the International rules 

 of zoological nomenclature, has already been pointed out by Schneider ('98). 



Inclusion in the above synonym of the Aglaja baerii of Eschscholtz ('25) is 

 necessary, because the figure of that form, especially of the posterior nectophore, 

 shows clearly that it was in reality A. tetragona. 



Plethosoma crystalloides Lesson, is a combination of the siphosome of 

 some Physophorid, with an Abylid; Huxley long ago pointed out that the latter 

 was almost certainly A. tetragona, with which Lesson's figure agrees very well. 



Neither Chun ('97b) nor Lens and Van Riemsdijk (:08) have placed the 

 Aglaisma peniagona of Leuckart ('53) here. Leuckart's figures show that his 

 specimens were young Abylopsis, either .4. tetragona or A. eschscholtzii ( = quin- 

 cunx). Unfortunately the one feature absolutely distinguishing the two, i. e., 

 the canal-course of the posterior nectosac, is not shown. But the shape of the 

 posterior nectophore suggests identity with the former rather than with the 

 latter. In including here the .4. pentagona of Huxley, which Haeckel ('88b) 

 and Chun ('97b) considered a separate species, I follow Lens and Van Riemsdijk, 

 and I need only add that examination of Huxley's figures has convinced me that 

 they were correct in thinking that Huxley's specimens were merely small examples 

 of A. tetragona. Aglaismoides elongata Huxley, agrees according to the figures, 

 with the many accounts of the "cuboides" Eudoxid of A. tetragona, and differs 

 correspondingly from the Eudoxid of A. quincunx. 



It is probable that the A. huxleyi of Agassiz and Mayer (:02) also belongs 

 here, especially since I have found specimens of ^4. tetragona in the material on 

 which their report was based. But in as much as their figure shows canals 

 of the ordinary Abylid type in a specimen otherwise resembling A. tetragona, 

 the identification remains doubtful. 



The present large and excellently preserved series allowed me to make a 

 thorough comparison between this species and A. eschscholtzii, with the result 

 that I am convinced the two species are entirely distinct. 



Several adequate accounts of ^4. tetragona have been published. It is not 

 likely to be confused with any species except A. eschscholtzii, and from the latter 

 it can readily be distinguished by its very large size; its relatively much longer 

 posterior nectophore (between four and five times as long as the anterior one), 

 and especially by the structure of that organ. The asymmetry of the nectophore, 

 and the diagnostic features of the hydroecium are mentioned in the account 

 of A. eschscholtzii (p. 228). The pecuUar arrangement of the canals of the pos- 



