PALEOPNEUSTES RYSTRIX. 6] 



surrounded by four or 6ve secondaries with the same scattering of distant 



miliaries. 



The secondary spines of the abactinal surface are small, Blender, sharp, 

 Bhort, Blightly curved, about •"> mm. in length ; the delicate iniliarj spines - are 

 .,!„,,,, half thai length. Toward the ambitus the tuberculatum become 

 closer, and on the actinal side (PL XVIII. Fig. 5) the primary tubercles 

 are Bomewhal 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 tuberculatum extends also over the ambulacral plates on the actinal 

 aide, so thai in this species there is nothing of the brdad bare ambulacra! 

 avenues, extending from the ambitus to the actinostome found in other 

 species of this and allied genera. On the actinal side the spin,- are pro- 

 portionally smaller, more slender, shorter, slightly curved and spathiform, 

 as in other Spatangoids. 



The phyllodes i PL XVIII. Figs. 5, 6, Ti are remarkably prominenl in this 

 species, and the actinostome is larger in proportion to the diameter of the 

 tesl than in P.cristatus. In a specimen measuring L 15 mm. in length, the 

 actinal opening measures 25 mm., while in a specimen of /'. crisiatus measur- 

 ing in nun. in length the actinostome measures only 27 mm. The other 

 principal differences of P. hystrix from P. cmtatiu are seen in the abactinal 

 system and the ambulacra! petals. There are four genital openings; hut 

 j u some specimens the left anterior genital is m.i as fully developed as 

 the others. It is therefore probable that in some cases we may find only 

 three genital pores, as in P. cmtatus. The ambulacral plates are compar- 

 atively higher and wider than in P. cristatus ; 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 plate-. 

 This causes the semi-petaloid part of the ambulacra to diverge more rapidly 

 than in the other Bpecies of the genus, the pair- of pores at the lower 

 extremity of the petals being nearly twice as distant as in /'■ cri 

 There is considerable variation in the size of the pores of the petaloid 

 ambulacra; a- a general rule, on the outer rows, the pores are comma- 

 shaped and much larger than the comparatively small pores of the inner 

 rows, hut in other specimens this difference is not so striking. When alive 



the color of the test varied -really; it IS in some specimen- of a rich light 



chocolate color, from which stand out in striking contrast the long yel- 



