ECHINARACHNTUS EXCENTRICUS. 525 



odd anterior petal is by far the longest, acuminate, with a broad interporiferous 

 space, enclosed by a narrow poriferous zone. The anterior pair are somewhat 

 shorter, rounded at the extremity ; the interporiferous area narrow, bounded 

 by broad poriferous zones. The posterior pair of ambulacra are still shorter, 

 scarcely more than half as long as the odd anterior ambulacrum, with broad 

 poriferous zones, open and rounded at the extremity ; the anterior poriferous 

 zone not regularly arched like the posterior zone, but decreasing very rapidly 

 in width near the apical system. The vertex is eccentric posteriorly ; tin- 

 test slopes gradually from the thin edge to the extremity of the petals, and 

 then it is regularly arched. The lower surface is flat ; mouth nearer the 

 posterior edge, corresponding to vertex ; the anus is placed at a distance 

 from the edge, from two to three times its diameter The ambulacral furrows 

 are very unequally developed ; they cover with their ramifications the whole 

 of the posterior half of the actinal surface, but extend only a little more 

 than half-way between the mouth and the edge of the test in the anterior 

 half. Portions of the ambulacral furrows extend even beyond the edge to 

 the abactinal part of the test, reaching, in the three posterior interambulacral 

 spaces, the level of the ambulacral petals, and in the petals sending two 

 branches, one of which extends nearly to the abactinal system in the inter- 

 poriferous zone of the two pairs of petals. The ambulacral furrows are 

 broad and deep ; near the somewhat sunken actinostome they spread fan- 

 shaped for a short distance, and then send out two branches, which run in 

 continuation of the direction of the outer line of the furrow, and then run 

 nearly parallel towards the edge of the test ; each main furrow sending oft" 

 a branch at right angles, extending into the interambulacral area, which 

 trends towards the edge of the test at its extremity. Numerous short spurs 

 are sent oft* from the furrows at right angles generally to the general trend 

 of the branches or main furrows, which in their turn may branch again. The 

 ramifications of the furrows are very variable, specimens of the same size 

 frequently possessing numerous small offshoots for the whole length of the 

 furrows and of their branches, while in others we have only the main 

 furrows and the principal branches, with here and there an occasional short 

 spur. The anterior ambulacral furrow, which is so much less developed than 

 the others, branches but once, and frequently remains simple at its extremity, 

 which does not extend to more than two thirds of the distance from the 

 actinostome to the ediye of the test. The tubercles of the actinal side are 

 much larger than those of the abactinal side ; they are especially large 



