Vol. 55.] ECHINOIDEA AND OPHIUKOIDEA. 701 



existed with the usual arrangement, more evident signs of them 

 might be naturally expected. The specimen is, however, somewhat 

 overfolded and overthrust, so that additional ambulacra may easily 

 exist concealed. 



The breadth of an ambulacral area is 3 '5 mm. ; it is composed of 

 two rows of narrow plates bearing double pores (fig. 5). The 

 length of a plate is 1*75 mm., 



its breadth about 0*32 mm., so Fig. 5. — Amhulacral and adjacent 

 that on the average there are interamhidacral plates of 



two ambulacral to each adjacent Myriastiches gigas, gen. et 



interambulacral plate. The pair sp. nov. ( x about 9). 



of pores is situated in that moiety 

 of the plate which helps to form 

 the median ambulacral suture. 

 The pore does not seem to be com- 

 pletely defined by its own plate, 

 an adjacent plate taking part in 

 its boundary. The character of 

 the sutural line between the 

 ambulacra and the interambu- 

 lacra is inconstant : sometimes it 

 presents itself as an approxi- 

 mately straight line, the sides 



of square interambulacral plates meeting the truncated ends of the 

 ambulacral ; sometimes it is a zig-zag, corresponding to the angles 

 made by adjacent hexagons when the interambulacral plates acquire 

 the hexagonal form. 



A hollow cast, lying in the midst of the flattened sac, appears to 

 represent a single strong jaw, like that of an Echinoid : of the 

 apical region there is no indication. 



In the characters of the ambulacra Myriastiches makes an approach 

 to the extinct Cystocidaroida of Zittel ; but if, as seems natural, the 

 genus is to be included in that group, the diagnosis of the order as 

 given by Dr. Gregory may have to be amended so as to admit of the 

 reception of forms unprovided with spines. 



The differences between Myriastiches and the two other genera — 

 Palceodisciis and Echimcystis — which so far form the contents of the 

 Cystocidaroida, are more than generic, and require the institution of 

 a new family, the Myriastichidge, which may be thus defined: — 

 Cystocidaroida in which the interambulacral areas are composed of a 

 large number of rows of minute square plates, and the ambulacra of 

 two rows of small plates bearing double pores suturally situated. 



Genus : Myriastiches, with the characters of the family. 



Pal^odiscus fekox, Salter. (Figs. 6-11, pp. 702, 703, & 704.) 



The specimens in the Oxford University Museum are in the usual 

 state of casts, only occasionally presenting remains of the original 

 calcareous test and spines. They afford views of the ventral surface, 

 sometimes from within, sometimes from without ; the dorsal surface 

 is rarely exposed, and none of the specimens exhibit the apical 



