426 COLOBOCENTROTUS MERTENSH. 



Colobocentrotus Mertensii 



Colobocentrotus Merteusii Br., 1835, Prod. An. Dos. 



PI III d . f. 4-5. 



Distinguished at once from C. atratus by the smaller disconnected spines of 

 the abactinal part of the test, which do not form a close pavement as in that 

 species. The spines terminate in an irregularly spherical head, more or less 

 separated by the miliary spines crowded in between the primary spines. 

 The spines of the ambitus are more slender, flattened, and not cylindrical 

 or club-shaped, as in C. atratus. The spines of the lower surface of the test 

 are quite short, slender, cylindrical, and more uniform in size than those of the 

 actinal surface of C. atratus. The color of the test, covered with spines, is 

 bluish-green above and light reddish-hrown on the actinal surface. The actinal 

 membrane is thin, having a lew irregularly scattered plates carrying short 

 slender cylindrical spines; these small spines are somewhat more numerous 

 on the ten large buccal plates. 



When denuded, the tesl of this species is in striking contrast to the other 

 species of the genus; and at the time of writing the Synopsis of Stimpson's 

 No. Pacific Echini I separated this species as a distinct genus on account of the 

 four rows of primary tubercles of the ambulacra! zone. This alone is not a 

 character of sufficient value to maintain a generic division, and the genus 

 has here been accepted as originally proposed by Brandt 



The test is materially thinner than that of C. atratus. the outline more 

 circular and less elliptical ; the actinal surface is quite Hat, slightly concave; 

 the test in profile is conical, rising quite gradually from the ambitus, and not 

 gibbous or swollen near the ambitus, as in C. atratus. The primary tubercles 

 immediately on the edge of the ambitus are by far the largest, diminishing 

 suddenly in size towards the actinostome ; the narrow median ambulacral 

 spaces left between the broad flat petaloid base of the poriferous zone are 

 filled by two rows of small tubercles, with here and there an irregular 

 secondary in the midst of the poriferous zone. In the interambulacral space 

 there are three pyramidal lines of tubercles starting from the ambitus and 

 terminating in three single vertical rows of secondaries near the actino- 

 stome. The actinostome is larger, the actinal cuts more marked, and the 

 lips more prominent than in C. atratus. 



The whole test immediately above the large tubercles of the ambitus up to 

 the abactinal system is covered by primary tubercles of uniform size, only 

 slightly smaller in the immediate vicinity of the abactinal system ; the same 



