566 SPATANGUS PURPUREUS. 



space towards the edges of the interambulacral plate. The large tubercles 

 extend nearly to the anal system in the odd interambulacrum : in the lateral 

 posterior interambulacra they reach the ambitus, or nearly so. in the anterior 

 part of the space ; in the (interior lateral ambulacra they extend to the am- 

 bitus. Adjoining the whole length of the anterior groove to the ambitus, 

 there are prominent tubercles, closely packed, which gradually pass into the 

 small, uniform-sized, secondary, perforated, indistinctly crenulated tubercles, 

 which cover the whole of the abactinal part of the test, including the inter- 

 poriferous zones ; the miliaries are minute, forming irregular arcs running 

 at a distance round the base of the small, well-separated secondaries. The 

 miliaries are more numerous in the anterior groove, in the interporiferons 

 space, than in the corresponding space in the petals. As the secondaries 

 approach the ambitus they gradually increase in size, become more dis- 

 tinctly crenulated, and pass into the larger primary tubercles of the actinal 

 side ; the miliaries forming more or less regular circles round the bare 

 scrobicular area ; these increase regularly in size in the lateral ambulacra, 

 till they reach the broad bare ambulacra! avenue-, extending from the 

 month to the ambitus. — the anterior ones to the very edge of the subanal 

 fasciole, and the others to the edge of the test. The anibulacral avenues 



are covered by minute distanl miliaries. The actinostome is crescent- 

 shaped, broad ; posterior lip not remarkably prominent. The actinal plas- 

 tron is elongate, triangular, with a slight keel near the central part. It 

 is covered by large tubercles, gradually becoming smaller towards the me- 

 dian line. The posterior edge of the test, including a part of the actinal 

 surface, is occupied by a large transverse subanal plastron, edged with 

 a broad fasciole, with a deep re-entering angle below the transverse, 

 elliptical, pointed anal system. The tuberculatum of the subanal plastron 

 is largest in the central part of the corresponding loops formed by the fas- 

 ciole, on either side of the median line. The anal system is crowded b} r an 

 outer row of nearly rectangular plates, succeeded by concentric and more 

 irregularly shaped plates, gradually becoming smaller towards the anal 

 opening : these plates carry a few miliaries. 



The secondary tubercles carry short, slender, pointed, curved spines, 

 longitudinally anil transversely striated. The larger spines carried by the 

 larger primary tubercles of the abactinal side and by the larger tubercles of 

 the actinal side differ only in size from the smaller ones; they are lighter 

 colored, usually of a yellowish tinge, while the smaller spines are of uni- 

 form and darker tint. 



