Echinoidea from Southern Chile — PAWSON 209 
FIGS. 1-3. Arbacia dufresnei and Pseudechinus magellanicus. 1, A. dufresnei, abnormal apical system with 
seven anal valves. 2, P. magellanicus juvenile, showing radiating epistromal ridges in an ambulacrum. 3, P. 
magellanicus , abnormal apical system with ocular IV insert. Abbreviations: amb., ambulacrum; oc. Ill, ocular III. 
adults 12-15 mm in diameter. His material was 
collected in July, while the present specimens 
were collected between November and February. 
The apical systems of 50 specimens were ex- 
amined. A specimen 6 mm in diameter has two 
large suranal plates instead of the usual one, and 
these plates almost fill the periproctal space. In 
almost all cases, the anterior ocular plate (oc. 
Ill) is strongly exsert and oc. I is insert, while 
the anus tends to occupy a position posterior in 
the periproct, near oc. I. Some specimens show 
other ocular plates near the posterior end of the 
apical system to be almost insert (oc. II and 
V), while in a specimen of 27 mm h.d. oc. I 
and II are broadly insert and oc. V is almost 
insert. The fact that oc. Ill remains the most 
strongly exsert of all the ocular plates agrees 
with the observations made by Fell (1963) on 
the tendency for the anus to migrate posteriorly, 
and the effects of this tendency on the plates 
of the apical system. Another specimen 23 mm 
in diameter has an unusual apical system (Fig. 
3 ) in which only oc. IV is insert, and while the 
anus is as usual excentric, it does not lie in its 
normal posterior position, but nearest oc. IV. 
Oculars I, II, III, and V are here strongly exsert. 
Small specimens were examined for traces of 
a temnopleurid epistroma, which has already 
been found in some other members of the genus 
Pseudechinus (see above). Epistroma is def- 
initely present in P. magellanicus, at least in 
juvenile specimens. The smallest specimen in 
the collection (4 mm h.d.) shows weakly de- 
veloped ridges radiating from the areoles of 
primary and secondary tubercles, especially in 
the ambulacra, near the apical system (Fig. 2). 
