L»24 W. KEEPING OX 1'HE GE>TS PELAXECH1XI •. 



54. On Pelajtechhtus*, a new Genus of Sea-urchins from the Coral 

 Bag. P>y Waetee Keeping, Esq., B.A., F.G.S., Professor of 

 Geology in the University College of Wales. (Head June 19, 

 1878.) 



[Plate XXXIV.] 



This is a Sea-urchin of more than ordinary interest, and remark- 

 able for possessing an unusually developed system of overlapping 

 plates around the mouth-opening. Such a structure is now well 

 known in the deep-sea Asthenosoma (or Calveria). where it has been 

 compared to the whole tests of some flexible Echini from the older 

 (Palaeozoic) rocks. I believe that the whole test of this Urchin 

 also was flexible, though not nearly to the same astonishing degree 

 as in Asthenosoma. 



Dr. Wright, in 1S55, described this species from very fragmentary 

 portions under the name Hemipedina coralltiiaf ; but since that date 

 the more perfect specimens described below have been found, and 

 the characters seen in these demand their wide separation from Hemi- 

 p dina, and establish a new genus whose affinities are rather with 

 the Echinothurido!. 



I have only two specimens to work at, not being able to find 

 those mentioned by Dr. Wright on page 164 of his Monograph (Pal. 

 Soc). Both of these are, however, very good. One was found by 

 my father at Calne in IS 00, and is now in the Wcodwardian Mu- 

 seum, Cambridge : the other, from the same locality, is in the 

 cabinet of Dr. Wright. They have been kindly lent to me for 

 description. 



I. The Woodwaediax Musettm Specimen. 



The Woodwardian Museum specimen is a fine Urchin, 4 inches in 

 diameter, with the whole of its oral hemisphere beautifully exposed 

 (PL XXXIV. fig. 1). It is circular in contour, and has subsided, 

 it may be by its own natural weight helped by subsequent rock- 

 pressure, to the proportions of an ordinary plum bun. In the centre 

 is seen the dental apparatus surrounded by a zone of scale-like actinal 

 (mouth-membrane) plates ; and more externally are the five ambula- 

 cral and five inter ambulacial areas of the ordinary Echinoid character. 



All the plates are unusually thin, and all, including the squamous 

 actinal plates, are covered with radiating series of tubercles. Spines 

 are abundant, but small, only a few of the primary set being pre- 

 served. 



The interambulacral areas (fig. 3) are broad and rapidly expand- 

 ing (measuring | inch at the oral extremity and 1|- inch at the 

 equator) ; each is ornamented with six or eight radiating series of 

 primary tubercles of moderate size, which diminish slightly towards 



* TliXavcs, a soft cake offered to the gods. 



t British Fossil Echinodermata Oolitic (Pal. Sec), p- 163, t. xii. f. 1. 



