THE GENUS COLOBOCENTEOTUS. 11 



Colohocentrotus Sfimpsoni. These figures show how the distant openings for 

 the pedicels slant across the thickness of the test to come together and form 

 the pairs of pores. 



On PI. 41, fig. i, is given a figure of the left anterior interambulacrum of a 

 specimen of Colob. Stimpsoni, measuring with its spines 88 mm. in diameter 

 with 14 and 15 coronal plates. At the ambitus on the abactinal side there 

 are 12 rows of primary tubercles which form disconnected vertical rows ; at the 

 tenth plate there are but five rows and at the third plate from the apex 

 only three. The tubercles diminish but slightly in size from the ambitus 

 towards the abactinal system and the intertubercular space is well filled with 

 miliaries. Below the ambitus on the actinal side are found the largest of 

 the primary tubercles forming two horizontal lines, the lower one of which is 

 the largest, extending across the fifth and sixth plates from the actinal sys- 

 tem. These tubercles carry the large flattened radioles truncated at the tip 

 which form the ring of larger radioles round the base of the abactinal part of 

 the test. 



Between the row of large tubercles and the actinal system the interambu- 

 lacral plates on each side of the poriferous field become much narrower 

 (Pis. 41, fig. 1 ; 38, fig. 1 ; 34, fig. 1). There are only four indistinct verti- 

 cal rows of small primary tubercles. The rest of the plates is filled with 

 still smaller primary tubercles and patches of miliaries irregularly arranged. 



The actinal lips are short but well marked. The lower part of fig. 1, PI. 41, 

 shows the distortion which takes place in the outline of the large primary 

 tubercles below the ambitus owing to the great compression due to the forma- 

 tion of the sharp angle of the test at the ambitus. Fig. 3, PI. 41, shows one 

 of the most distorted of the primary tubercles on the fifth plate from the 

 actinostome, the mammary boss, and the scrobicular area being pushed out of 

 shape. Fig. ^ shows the same tubercle in profile. Fig. 2, PI. 41, shows the 

 more regular primary tubercles of the abactinal part of the test in which 

 the boss and the scrobicular area are but slightly distorted, the boss and 

 area being usually circular, PI. 41, fig. 5. The grouping of the miliaries does 

 not seem to follow any rule. PI. 41, fig. 6, shows those of one of the plates 

 near the actinostome of the right posterior ambulacrum. U'vtyfev 



In a specimen measuring 88 mm. in diameter, including the spines, it is pos- / 



sible to trace the twelve vertical rows of the primary radioles of the interam- 

 bulacral areas. In PI. 39, fig. 3, the dotted lines show the twelve rows of the 

 right posterior interambulacrum and the four vertical rows of the right poste- 



