78 



TH. MORTENSEN, 



(Schwed. Sudpolar-Exp. 



Fig. 9. AGASSIZ does not give a detailed explanation of this or the other figures, 

 and I am unable to grasp his meaning fully from the numbers put on the plates. 

 But there can be no doubt of what is considered the Ocular III, it must be that 

 with the tentacle; inside this plate are seen 3 smaller plates, two outer paired, and 

 one inner single. What are these plates? In the figure 307 they are seen to be 

 provided with pores; in the figure 308 these pores have metamorphosed into tu- 

 bercles, but judging from PI. 99. Fig. 7 they would much sooner represent tubefeet. 

 This would mean that they represent the Ocular III and two ambulacrals, and the 

 comparison with the figure 16 leaves no doubt that this is the true explanation. 

 The alleged ocular plate outside must be incorrectly drawn, and likewise the odd 

 tentacle has got a wrong place. When this has been made clear the identification of 

 the rest of the plates is easy enough; it will be sufficient ro refer to the two figures. 



Regarding the actinal side of the test in the figures quoted of the »Panamic Deep 

 Sea Echinh I would only remark that Professor AGASSIZ' surprising statement of 

 the sternum forming »almost a true meridosternum* (p. 212) is evidently due to a 

 misconception. It is, as should be expected, a true amphisternum, the two sternal 

 plates being quite equally developed in the anterior end; the fact that one is slightly 

 longer than the other at the aboral end, a character well known in different types 

 of amphisternum, especially the more primitive, is likewise only what should be 

 expected. (Cf. »Ingolf» Echinoidea. II. p. 84.) 



The next stage observed is that of the fully developed young, ready to leave 

 the marsupium, 2.5 mm. in length* (Figs. 17 — 18). The most important change to 



i8. 



Figs. 17 — 18. Abatus cavernesus, 2.5 mm. Abactinal (17) and actinal side (18). 



* In a fragment of a large specimen, probably of Abatas cavernosas, from the Magellan Strait (the 

 Hamburg Museum) I find the young ones in the marsupium considerably larger, 3.5 mm. in length. The 

 transverse band of the fasciole has not yet appeared in these specimens. 



