PALEOPNEUSTES HYSTRIX. 61 
surrounded by four or five secondaries with the same scattering of distant 
miharies. 
The secondary spines of the al)actinal surfece are small, slender, sharp, 
short, slightly curved, about 5 mm. in length ; the delicate miliary spines are 
about half that length. Toward the ambitus the tuberculation becomes 
closer, and on the actinal side (PI. XVIII. Fig. 5) the primary tubercles 
are somewhat smaller and arranged in three irregular horizontal rows, quite 
closely packed on the interambulacral plates, with only a few scattered sec- 
ondary tubercles and less numerous miliaries than on the abactinal surface. 
This tuberculation extends also over the ambulacral plates on the actinal 
side, so that in this species there is nothing of the broad bare ambulacral 
avenues, extending from the ambitus to the actinostome found in other 
species of this and allied genera. On the actinal side the spines are pro- 
portionally smaller, more slender, shorter, slightly curved and spathiform, 
as in other Spatangoids. 
The phyllodes (PI. XVIII. Figs. 5, 6, 7) are remarkabl^^ prominent in this 
species, and the actinostome is larger in proportion to the diameter of the 
test than in P. crisUitus. In a specimen measuring 115 mm. in length, the 
actinal opening measures 25 mm., while in a specimen of P. crisiaius measur- 
ing 144 mm. in length the actinostome measures only 27 mm. The other 
principal differences of P. hjsirix from P. cridahis are seen in the abactinal 
system and the ambulacral petals. There are four genital openings ; but 
in some specimens the left anterior genital is not as fully developed as 
the others. It is therefore probable that in some cases we may find only 
three genital pores, as in P. cristahis. The ambulacral plates are compar- 
atively higher and wider than in P. cristahis; in addition, the pairs of 
pores are placed nearer the outer edges of the ambulacral plates than in 
P. cristatus, where they are situated nearly in the centre of the plates. 
This causes the semi-petaloid part of the ambulacra to diverge more rapidly 
than in tbe other species of the genus, the pairs of pores at the lower 
extremity of the petals being nearly twice as distant as in P. crisfdns. 
There is considerable variation in the size of the pores of the petaloid 
ambulacra ; as a general rule, on the outer rows, the pores are comma- 
shaped and much larger tlian the comparatively small pores of the inner 
rows, but in other specimens this diflFerence is not so striking. When alive 
the color of the test varies greatly; it is in some specimens of a rich light 
chocolate color, from which stand out in striking contrast the long yel- 
