PILEMATECHINUS RATHBUNI. 165 



Pilematechinus Rathbuni A. Ag. 



Cystechinus Rathbuni A. Ag., Bull. M. G. Z. 1898, XXXII, No. 5, p. 79, Plate X. 



Plates 81-86. 



A number of specimens in excellent condition came up in the trawl from 

 the two stations at which this species was procured. While the test of 

 some of the specimens came up more or less indented, yet a few preserved 

 their shape, and none showed such extremes of flexibility as in C. vesica; 1 we 

 are thus able to give figures of the outline of the group of Cystechinus 

 to which this species (Pis. 81 ; 82) and C. vesica belong. The test of 

 P. Rathbuni is much thinner than that of C. Loveni, but somewhat thicker 

 than that of C. vesica. The coronal plates are frequently split by longi- 

 tudinal or transverse cracks due to the sharp folds of the sides of the test 

 (Pis. 82; 85, fig. 5) or differences in the thickness during the growth of the 

 plates (PI. 86, fig. 4). 



Pilematechinus Rathbuni is a most striking sea-urchin. Its enormous 

 height compared to its diameter is unique among recent Spatangoids. When 

 alive the specimens varied from a brilliant dark violet to a light claret color. 

 The specimens collected were nearly of the same size : 95 mm. in greatest 

 length, 82 mm. in height, and 75 mm. at its greatest diameter (Pis. 81 ; 82). 

 It is interesting to note that among the deep-sea types of Echini we find 

 also the extreme elongated forms of Pourtalesia and Aerope and the flattened 

 Echinothuria 1 . Seen from above (PI. 81, fig. 2) the outline is but slightly 

 elliptical, with blunt anterior and posterior extremities. The straight edges 

 of the coronal plates give it an angular outline at the ambitus. The actinal 

 side (PI. 81, fig. 1) is quite flat having a somewhat pentagonal depression 

 around the actinostome, a sharply rounded ambitus (PI. 82) and a depressed 

 subcentral actinal system. The actinostome is about 40 mm. from the 

 anterior edge of the test, and 55 from the posterior extremity. 



The whole actinal side is thickly covered with small primary tubercles 

 (Pis. 81, fig. 1 ; 83, fig. 1), though many of the plates near the ambitus show 

 only traces of the former primary tuberculation. On the actinal side the 

 plates are thickly covered with small iniliaries. The prominent actinal 

 tuberculation is in marked contrast with that of the abactinal part of 

 the test, where the coronal plates carry a less crowded pavement of small 



1 "Challenger" Echinoidea, PI. XXXV, figs. 1-4. 



