STRONGYLOCENTROTUS MEXICANUS. 447 



excluded from the anal area. In medium-sized specimens but one ocular 

 plate reaches the anal system. Genital ring broad, plates triangular, madre- 

 poric genital much the largest, genital openings moderate ; ocular plates 

 triangular, with rounded basal corners. 



Two principal vertical rows of primary tubercles both in the ambulacral 

 and interambulacral regions ; median ambulacral space filled with small 

 irregularly arranged secondaries forming a zigzag vertical line. In the 

 interambulacral space each primary row is flanked by a second vow of 

 smaller tubercles; of these the median rows coalesce and run irregularly to 

 the apex, the exterior row reaching the apex as small secondaries. 



Poriferous zone half as broad as the ambulacral zone, generally five pairs 

 of pore.-; to each arc ; the arcs separated by very minute tubercles, from 

 two to three, forming irregular oblique lines. The arcs of pores are some- 

 what oblique and but slightly open. The main tubercles of the ambulacral 

 and interambulacral spaces are surrounded by irregular circles of secondaries 

 and few miliaries forming lines between them. The coronal plates are nar- 

 row. The lower surface but slightly arched ; the actinostome small ; the 

 buccal membrane thin, with a few small distinct plates in the continuation 

 of the ambulacra. Spines long, slender, varying in color from olive to violet 

 purplish or even yellowish tint, tipped with yellow and all intermediate 

 shades. 



European ; Atlantic ; Mediterranean ; Azores. 



SSrongylocentrotus mexicanus 



! Toxocidaris mexicana (A. Ag.). 1863, Bull. M. C. Z. 



! Strongylocentrotits mexicanus A. Ag., 1872, Rev. Eeh., Pt. I. p. 165. 



This species is distinguished from S. tuberculatus, to which it is allied 

 most closely, by its more compact anal system, narrow genital ring, and 

 large madreporic body. The poriferous zone above the ambitus is narrow", the 

 arcs of pores being nearly vertical. The actinostome is relatively large in 

 this species. In the ambulacral region the two main vertical rows of pri- 

 mary tubercles are placed close together, the median vertical row is reduced 



