PILEMATECHIXUS. 



163 



of nearly nniform size, tlie outer row being somewhat larger, with an inner 

 row of small elongated plates round the actinostonie (PI. SO, fig. ]). In 

 the larger specimen the anal system has become greatly elongated with a 

 prominent outer row of pentagonal plates, the central part being covered 

 by smaller polygonal and elongated plates, all except those near the anal 

 opening carrying a number of miliaries (PI, 80, fig. 7). The plates of the 

 anal system have not yet assumed the imbricating arrangement they take 

 in older stages (" Challenger " Echinoidea, PI. XXIX% fig. 20) or in C. 

 clypeatus (PL XXXV", fig. 11). 



Station 3415, off Acapulco, 1879 fathoms. Lat. 14' 46' N. ; Long. 98' 

 40' W. Bottom temperature 36''. Br. m. glob. ooze. 



PiLEMATECHINUS A. Ag. 

 Cystechinus pars A. Ag. Bull. M. C. Z. 1898, XXXII, No. 5, p. '79. 



In the preliminary Report on the Echini of this Expedition, I referred 

 the extraordinary sea urchin described as Cystechinus Rathhuni to the genus 

 Cystechinus. Subsequent examination has led me to propose a new genus 

 for this interesting species, for the following reasons. 



The primary coronal plates adjoining the 

 actinal system are smaller than the labium, 

 Fig. 235, and are followed by a single plate. 

 It is possible that the labium is made up of 

 two plates and then followed by the regular 

 succession of plates belonging to the right 

 and left odd interambulacral zones (PL 85, 

 figs, i, 2). Both in C. Wijvillii (PL 80, fig. 1) 

 and C. Lovmi Fig. 237 (PL 78, fig. 3) a 

 single sternal plate follows the labium. Fig. 

 236, as in Urechinus (PL 74, fig. 6) ; while 

 in C. Rathhuni the labium is followed by the 

 plates of the two zones of the odd interambulacrum (PL 85, figs. 1, 2). 



So radical a difference may justify us in separating generically, as Pile- 

 matechinus, C. Rathbani from C. Loveni and associating it in a new genus 

 with C. vesica, in which the structure of the plastron is like that of C. 

 Rathhuni, it having a high thin test and the anus well above the ambitus. 



If the labium consisted of two plates the sternum would be constructed 

 like that of Offaster, but from analogy of the young of Abatus it has the rudi- 



« 5 a 

 92 mm. 



Fig. 235. Pilematkchtsus R.*.THBUin. 



