ECHINOSTREPHUS. 457 



ECHINOSTREPHUS. 



Echinostrephus A. Ac, 1863, Bull. M. C. Z., I. 



The shape of the test is very striking ; unlike other Echini, the great- 

 est diameter is near the abactinal surface, which is flattened. The test slopes 

 gradually towards the actinostome ; this is small, with slight cuts. Be- 

 sides the unusual outline of the test, the peculiar abactinal system is very 

 characteristic. It is circular ; the ocular plates do not project beyond the 

 outline of the genital ring, but are wedged in between the genital plates 

 somewhat as in Cidaris. The anal system is of moderate size, covered by a 

 central row of large plates. The spines are long, slender, tapering, exceeding 

 in length the diameter of the test; they have the usual structure of the 

 spines of Echinometradae. The teeth present no peculiar features ; auricles 

 small, highly developed, connecting ridge but little prominent. 



Echinostrephus molare 



Eclunometra setosa Rumph., 1705, Amb. Rar. Kam. 



\ Echinostrephus molare A. Agass., 1872, Rev. Ech., Pt. I p. 119. 



PL V a .f. 10-12.; PI VI. f. 20. 



Interambulacral region occupied by ten vertical rows of tubercles of uni- 

 form size, diminishing gradually in size from the point of greatest diameter 

 towards both poles. The coronal plates towards the actinostome are high, Avhile 

 they are narrow and elongate towards the abactinal system. Miliaries and 

 secondaries are irregularly scattered on the lower surface of the test, while on 

 the abactinal surface they are arranged round the primary tubercles. There 

 are two vertical rows of tubercles in the ambulacral space fully as large as 

 those of the interambulacral area. They are flanked by an exterior vertical 

 row of small tubercles extending into the poriferous zone ; the median space 

 is occupied by a few secondaries and miliaries arranged round the primaries. 

 There are from three to four pairs of pores in each arc, the majority having 

 but three pairs. The spines are long, slender, tapering, but gradually vary- 

 ing in color from flesh-color to dark-violet. The whole abactinal system 

 is covered by distinct but well-separated secondaries. The buccal membrane 

 is thickly covered by minute longitudinal plates only in the prolongation of 

 the ambulacra; rest of membrane bare. Madreporic body distinct, well 

 marked, but, as in Cidaridae, the madreporic genital plate is not more 



