928 W. KEEPING ON THE GENUS PELANECHINFS. 



to correspond with the primary tubercles. One is first reminded by 

 them of the actinal system of overlapping plates ; but these latter 

 are not so large, and are less regular in thickness; especially, too, 

 the proportions of the exposed to the overlapped portions in the two 

 forms of plates do not at all agree*. It may be that they formed 

 an extra internal supporting skeleton to this fragile test, the buccal 

 membrane having developed inwardly to extend up over that part 

 of the test nearest to the mouth and secreting these calcareous plates. 

 But additional evidence must be obtained from other specimens 

 before this can be proved. 



Some groups of irregularly perforated plates underlying part of 

 the ambulacral zone are probably parts of the internal surface of the 

 same zone crushed in from the apical side. 



Pelanechintjs, nov. gen. 



Generic characters. — Test thin, in form circular, depressed. Com- 

 posed of (1) transversely elongated coronal plates, (2) apical plates 

 surrounding the anus, and (3) an actinal system of imbricating 

 plates around tho mouth. Interambulacral areas narrow at the poles, 

 but rapidly broadening towards the ambitus, and bearing numerous 

 (six to eight) rows of primary tubercles. Interambulacral plates nar- 

 row, their contours rounded and slightly undulating. 



Ambulacral areas one third as broad as the interambulacral (at 

 the equator), supporting two ranges of primary tubercles. Poriferous 

 zones broad, the pores trigeminal in the equatorial region. 



Primary tubercles rather small, perforated, and mounted upon 

 elevated bosses with smooth uncrenulated summits; they are uni- 

 form over both areas. 



Peristome deeply notched. Actinal area extensive (about three 

 eighths of the whole test), covered with zones of large imbricating 

 scale-like plates, which are perforated for the tube-feet and bear 

 perforated tubercles. 



Spines small, hollow. Jaws large and powerful. 



The above characteristics will readily serve to distinguish Pcla- 

 nechinus from all other fossil genera yet known. None at all resemble 

 it in the nature of its actinal system of plates, except the Echinotlmria 

 of the Chalk ; and from this genus it differs widely. We shall find, 

 however, a much closer relationship between Pelanechinus and the 

 deep-sea Astlienosoma ( Calveria). Their general similarity of appear- 

 ance is striking, and these are the only two genera of Urchins yet 

 described as having a diademoid test associated with overlapping 

 buccal plates. Both are further characterized by their very thin 

 and narrow plates, their strong jaws, and hollow spines. 



Asthenosoma is well distinguished from our genus by its imper- 



* Only about one fifth of each plate is overlapped in the set we are now 

 considering, whilst about two thirds of the actinal plates are so covered. 



