MARETIA PLANULATA. 571 



tubercles, perforate, crenulate, increasing in number towards the ambitus, 

 arranged parallel to the upper horizontal sutures of the plates. The pri- 

 mary tubercles carry long, slender, curved spines, more than half the length 

 of the test. The other tubercles carry similar curved spines, but quite 

 short and slender. On the actinal side the posterior bare ambulacra! 

 avenues coalesce, forming a broad, bare actinal shield, slightly convex, 

 terminating posteriorly in a small triangular actinal plastron, covered by 

 secondary tubercles. The edge of the test is quite sharp, sloping towards 

 the sunken actinostome, and the sides of the bare ambulacral spaces. The 

 tubercles increase rapidly in size from the edge of the test inwards, and are 

 arranged in diagonal rows in the lateral posterior interambulacra. The actino- 

 stome is large ; the anterior lip is parallel to the posterior raised lip, on a 

 lower level, forming a narrow parallelogram, bent in the middle, with rounded 

 corners. The anterior phyllodes are well developed, extending to the edge 

 of the test ; the posterior phyllodes have but three or four pairs of parallel 

 pores, extending a short distance beyond the actinostome. The development 

 of the bourrelets in this species is quite marked. The bare actinal surface 

 has a few distant miliaries, carrying minute, slender, curved spines; the 

 spines of the large tubercles of the actinal surface are like the long spines 

 of the abactinal side. 



The coloration of this species is extremely variable, and has given rise to 

 the distinction of several species. It is (in alcohol) frequently of a uni- 

 form straw color, with darker spots at the base of the primary tubercles on 

 the abactinal side. In other specimens the abactinal part of the test is 

 light, surrounded by a dark violet-brown margin, running completely round 

 the edge of the test; the spines varying in color according to the position 

 in the dark or light areas. In other specimens the ambulacral petals alone 

 are colored violet-brown, with darker spots at the base of the primary spines. 

 In others, again, we have a combination of the marginal coloring and of the 

 dark ambulacral petals, forming irregular patches at the extremity of the 

 petals ; the coloring, when present, is usually strongest and most abundant 

 in the lateral ambulacra ; the odd ambulacrum is frequently only slightly 

 colored, or of a uniform yellowish tint in mottled specimens. The primary 

 spines are more or less intensely banded, according to the general tint of 

 the specimen. 



In large specimens the pores of the abactinal portions of the anterior 

 zone of the lateral ambulacra are more or less indistinct ; the pores of this 

 anterior zone are usually smaller than those of the other zones. 



