152 THE VOYAGE OF H.M.S. CHALLENGER. 



(1883). Not improbably too it may have been among the collections made by the 

 earlier " Willem Barents " Expeditions and by the " Vega "; though, as in the case of the 

 " Porcupine " specimens, it was not distinguished from immature individuals of Antedon 

 eschrichti. 



A careful study of all this material has convinced me, however, that the two forms 

 are very different in reality ; though, as I have pointed out above, Antedon quad rata 

 may in some sense be regarded as a permanent larval form of Antedon eschrichti. Its 

 first radials are not entirely concealed by the centro-dorsal, but appear above it as short 

 band-like plates (PL XXVI. figs. 1-3). The second radials have more sloping sides than 

 in the smaller forms of Antedon eschrichti, so as to be trapezoidal in general outline ; and 

 the axillaries have a blunter distal angle than in that type (PI. XXIV. figs. 10, 11). 

 The arm-bases are not tubercular, though the joints between the first and second syzygies 

 have the same backward projections on the sides which do not bear the pinnules that 

 occur in Antedon eschrichti. The relatively long quadrate shape of the arm-joints 

 immediately after the third syzygy is less marked in the Challenger specimens of Antedon 

 quadrata, the southernmost ones known (PL XXIV. fig. 2), than it is in the two obtained 

 further north by the " Triton " (PL XXIV. fig. 3), and in those from the Arctic Ocean 

 which have been figured elsewhere by Sladen ' and myself. 2 But the middle and outer 

 arm-joints of the two species are always distinguishable, those of Antedon eschrichti 

 being short, generally triangular, and much wider than long, till quite near the end of the 

 arm ; while the brachials of Antedon quadrata are obliquely quadrate and the length is 

 more nearly equal to the width. This is especially marked in the " Valorous " specimen, 

 and is no doubt partly due to its not being quite mature, as in the young forms obtained 

 by the Challenger (PL XXVI. fig. 1) ; but it is also very distinct in the larger examples 

 from the Barents and the Kara Seas. 



The other special mark of Antedon quadrata is the disproportion between the second 

 and the third pinnule, which has already been noticed by Sladen as distinguishing the 

 type from Antedon eschrichti (PL XXVII. figs. 9, 10, 12, 13). In the individuals of 

 the latter species which were obtained by the Challenger at Station 48, the third pinnule 

 is relatively much shorter than in the more northern forms. In large examples of 

 Antedon eschrichti from the Arctic Ocean it is of almost exactly the same length as the 

 second pinnule, as described by Sladen ; but in the West Atlantic representatives of the 

 type it is distinctly shorter (PL XXIV. figs. 8, 9). The southern forms of the two species 

 therefore approach one another in the characters of the pinnules, just as in those of the 

 arm-joints ; although the more northern varieties are entirely distinct in both respects. 



Not only is the third pinnule of Antedon quadrata altogether smaller than the 

 second, but its component joints, while fewer in number, are also different in their 



1 Op. tit., pi. vi. figs. 5, 6. 



2 Bijdragen tot de Dicrkunde, 1886, 13 Aflevering, vi., pi. i. fig. 6. 



