22 TH. MORTENSEN, (Schwed. Südpolar-Exp. 



The ambulacra and interambulacra show a somewhat larger number of secondary 

 tubercles than is the case in the smaller specimens of the typical form; each ambu- 

 lacral plate has two small tubercles inside the primary one. Otherwise no difference 

 is found herein. The median furrow is distinct, of the usual width. The peristome 

 has 10 — II ambulacral plates in each series; the 2 — 3 inner ones almost join those 

 of the adjacent series, the interambulacral plates thus scarcelj- reaching the mouth-edge. 



The pedicellariae are like those of the typical form, only in the large specimen 

 I find another peculiar form of small globiferous pedicellarise (PL XIV Fig. 9) which 

 I have not found in other specimens. — The radioles, the secondary spines and the 

 spicules are as in the typical form. — The apical system and the adjoining part of 

 the test is somewhat sunken in the larger specimen; young ones, however, were not ■ 

 found on it. 



Whether now these specimens are A. canaliculata or another nearly related 

 species, and whether the locality Altata is correct or not, they are in any case of 

 considerable interest (especially the larger specimen) through the curious reduction 

 of the apical plates, which reminds one of the Echinothurids, and has — as far as I 

 am aware — not hitherto been observed in Cidarids. It also sliows that the feature 

 of the interambulacral peristomial plates reaching the mouth-edge or not is no 

 reliable generic character. 



Wyville Thomson (Op. cit. p. 6-]) remarks on A. canaliculata that »this spe- 

 cies seems to acquire its full size during a single season. We dredged it at the close 

 of the breeding season, and we took no specimens intermediate in size between the 

 adult and the young». This suggestion is scarcely sufficiently well founded. First, 

 the breeding season is — so far as I can find — not known with certainty; we do 

 not know, whether the species does not breed all the year round, which is perhaps 

 not unlikely (comp. Stercchinns Neiimaycri, Echinoiden d. Deutschen Südpolar- 

 Exped. p. 71). Further, it is certainly no definite proof that no intermediate sizes 

 were taken in a few dredgings, especially in view of the fact that the Swedish 

 Expedition secured specimens of all different sizes in a single dredging (St. 43). 

 In case the species breeds all the year round or only a greater part of the year, it 

 must be impossible to conclude anything in this direction from the size of the 

 specimens caught, as then all the different sizes must be represented at any time 

 of the year. The question, however, would certainly deserve a close investigation; 

 we have otherwise no exact knowledge of the rate of growth of Echinoids. 



