106 PEOF. P. M. DrNCAK ON THE ANATOMY OF THE 



a plane perpendicular to the test, but that there is underlap of 

 one series of edges, and overlap of others. 



In JEcJiinotTirix calamaris the same structures occur as in 

 JE. Desori. 



V. AsTEOPTGA, Gray, 1825. 



This genus has species with the ambulacra projecting, and 

 ornamented with large tubercles similar to those of the inter- 

 radia. The pores are disposed in triple pairs. 



The sbape of the tubercles is rather like that of those of 

 Coelopleurus, and tbere is a bare median space near the apex in 

 botli genera. 



The definition of the genus is given in Dujardin et Hupe, Hist. 

 Nat. des Zooph. Echinodermes, Paris, 1862, p. 506, copied from 

 Desor, Synopsis, p. 83. Eacb of tliese authors states that the 

 distinction between Astropyga and Diadema consists in the former 

 having the pores in triple pairs, wbilst the latter has them 

 disposed in simple pairs, but forming arcs and undulating zones 

 around the ambulacral tubei'cles. 



In the ' E-evision of the Echini ' by A. A gassiz, the genus is con- 

 sidered critically (p. 417). It is noticed that the tubercles of 

 both areas are perforate and crenulate, and that the poriferous 

 zone of the ambulacra is broader than in Diadema, and nearly as 

 broad as the median ambulacral space. The pores are arranged 

 in four irregular vertical rows forming steps of pores of three 

 and one pair. In explaining the characters of Astropyga pulvi- 

 nata, A. Agassiz observes that the ambulacra have the vertical 

 rows of primary tubercles distant, frequently only every other 

 plate carrying a primary, the opposite being only a secondary. 



Reference is made to tlie general appearance of the ambulacra 

 of A. radiata on page 421 of the ' Eevision.' 



The genus is very critically considered in the Report on the 

 Echinoidea, ' Challenger ' Expedition, p. 72, under the Echino- 

 thuridse, and tbere are some important illustrations on plate x a. 

 figs. 8 & 9. 



Although the minute construction of the ambulacra is not 

 considered, there are important observations on the division of 

 the coronal plates above the ambitus, and the lapping of the 

 transverse sutures of the areas. The following statement relates 

 to a part of these researches in the anatomy of the test. " The 

 lapping of the coronal plates in the Echinothuridse is not so 



