68 THE VOYAGE OF H.M.S. CHALLENGER. 



resembles more that of Centrostephaniis and Aspidodiadema, and in the greater rigidity 

 of the test, as in Diadema ; the bare sunken median interambulacral space extending from 

 the apical system towards the ambitus, so characteristic of Astropyga, is in this genus nar- 

 row; it bears, as in Astropyga, small primary tubercles, forming two vertical rows along 

 the median ambulacral line. The primary tubercles of the interambulacral areas above the 

 ambitus are placed in the centre of the plates, and arranged in one principal vertical row, 

 with irregular rows of smaller secondary tubercles. As in Astropyga the actinal floor is 

 thickly covered with large primary tubercles, deep actinal cuts are present, the poriferous 

 zone is narrow, the pores are in pairs arranged in two vertical rows, the spines of the 

 abactinal surface are short and slender like those of Astropyga j)roper, while on the actinal 

 side they are more or less club-shaped, or trumpet-shaped, resembling somewhat the 

 actinal spines of A sfhenosoma and Phormosoma (but they are not tipped with a hoof as in 

 some species of these genera), with which both this genus and Astropnjga have many 

 points in common, forming a connection as it were between the Diadematicte proper and 

 the Echinothuridge. The primary and secondary tubercles are perforate, but not crenulate. 

 The primary and secondary ambulacral and interambulacral radioles are similar in structure, 

 with exceedingly fine verticillations, forming in older specimens a delicate longitudinal 

 striation. 



In Micropyga the long-headed pedicellariae closely resemble those of the Diadematidse. 



*Micropyga tuberctdata (Pis. VII., XXXIX. figs. 1, 2; PI. XL. figs. 26-28; PI. 

 XLIV. fig. 37). 



Micropyga tuherculata, A. Agassiz, 1879, Proc. Am. Acad., vol. xiv. \). 200. 



This is a large species, measuring no less than 200 mm. in diameter, the actinal surface 

 is flat (PI. VII. fig. 2), the ambitus making a sharp angle with it, and the test is regularly 

 arched towards the low flattened abactinal surface. On the actinal interambulacral spaces 

 the primary tubercles are arranged in a single horizontal row, occupying, with the exception 

 of a few miliaries, the whole of the coronal plate, they form a close pavement (PI. VII. figs. 

 2, 4), and increase uniformly in size from the actinal edge to the ambitus, making more 

 or less irregular vertical rows, radiating from the actinostome, there being from six to 

 seven rows at the ambitus. In the ambulacral areas the primary tubercles, arranged 

 in only two vertical rows, increase regularly in size towards the ambitus (PI. VII. figs. 

 2, 4), where they as well as the interambulacral tubercles are largest, and while occupying 

 there nearly the whole of the ambulacral plates between the poriferous zone, become 

 reduced on the abactinal surface to small secondary tubercles placed in the centre of the 

 ambulacral plates, which carry, besides, a few small miliaries or granules, occurring irregu- 

 larly on the plates (PI. VII. fig. 1, 7). The same rapid change takes place in the size of 

 the primary tubercles of the interambulacral areas of the abactinal surface (PI. VII. fig. 1) ; 



