UEECHINTJS GIGANTEUS. 153 



tionally flatter. On the actinal side the odd anterior ambulacrum is slightly 

 sunken near the actinostome, much less so than in the other species of the 

 genus. There is a mere rudimentary subanal fasciole in some specimens of 

 U. Naresiamts 1 (PI. 74, fig. 8), in others it is well developed. No trace of 

 such a fasciole could be detected in this species (PI. To, fig. 3). 



Seen in profile (PI. 72, fig. 3) the outline is regularly arched both towards 

 the anterior and posterior extremity, somewhat more gibbous at the anterior 

 extremity. The abactinal side passes most gradually into the ambitus and 

 actinal side. At the junction of the posterior interambulacrum and lateral 

 ambulacra the test forms a slight re-entering curve when seen in profile. 



This species is remarkable for the great length of the primary radioles 

 above the ambitus (PI. 72, fig. 2) ; in proportion to the size of the test they 

 are nearly as long as in Linopneustes and Paleopneustes. When covered 

 with spines, this species of Urechinus must have recalled small specimens 

 of Linopneustes; the spines do not increase in length with the larger speci- 

 mens. In the smaller specimen the length of the primary radioles on the 

 abactinal side along the sides of the test varied from 21 mm. to 8 or 9. 

 They are very slender, sharp-pointed, and curved. On the actinal side the 

 radioles are shorter, usually straight, and do not exceed 10 mm. in length, 

 the majority being from 1 to 6 mm. long. 



Above the ambitus the whole test is covered by minute miliaries (PL 73, 

 figs. 2, ->') scattered uniformly over the surface of the coronal plates except 

 on the posterior interambulacra, where they are more closely crowded. The 

 miliary tubercles carry either minute cylindrical spines or round-headed 

 pedicellaria) on stems of about f mm. in height. The primary tubercles 

 are placed pretty uniformly on the abactinal part of the test (PI. 72, figs. ;3, 3). 

 Both on the ambulacral and interambulacral areas there are from three 

 to seven primary tubercles on each plate ; they are more numerous and 

 more closely crowded together toward the ambitus (PI. 73, figs. .', J). In 

 the interambulacral areas of the actinal side the primaries become somewhat 

 smaller and are crowded on the plates (Pis. 72, fig. / ; 73, fig. 4.), forming 

 a close pavement ; this is specially the case in the odd posterior interambu- 

 lacrum (Pis. 72, fig. 1 ; 73, fig. 1). We find in Urechinus as in Cystechinus 

 bare circular spots (Pis. 73, figs. .', 3 ; 74, fig. 5) which represent primaries 

 and miliaries that have been resorbed and have disappeared, having been 

 dissolved as other carbonate of lime organisms dissolve at great depths. 



1 "Challenger" Echinoidea, Pis. XXX, fig. 19; XXX a , fig. 10 a . 



