248 ROBERT TRACY JACKSON ON ECHINI. 



height by 29 mm. in diameter. No ambulacral plates are preserved, so that the characters 

 of the ambulacra are derived wholly from the mold. The ambulacra at the mid-zone measure 

 about 7.5 mm. in width. As this is a measurement of the interior, on the exterior the width 

 would be somewhat less, because the interambulacral plates bevel over the ambulacrals on the 

 adradial sutures (text-fig. 239e). There are two columns of low plates in each ambulacrum. 

 The position of the plates is indicated in part by rather obscure impressions of sutures, but 

 mainly by the siliceous plugs which represent the casts of pores of the pore-pairs (text-fig. 239e) . 

 The pores of each pore-pair at the mid-zone are 1 mm. apart, and the distance from the outer 

 pore of one plate in a right half-area to the outer pore of the adjacent plate in a left half-area 

 is 6 mm. Four ambulacral plates equal the height of an interambulacral plate at the mid-zone, 

 as gathered from the number of pore-pairs in the mold opposed to an interambulacral plate 

 which is in place (text-fig. 239d). This number is similar to the number of ambulacral plates 

 opposed to an interambulacral in Miocidaris keyserlingi, as stated above, but it is much smaller 

 than the usual number in the adults of Recent cidarids (compare text-fig. 218, p. 191). In the 

 young of Recent cidarids, however, the ambulacral plates are higher and in very young speci- 

 mens may be even as high as the interambulacrals (Plate 2, fig. 2). 



The only interambulacrum which is entirely exposed to view measures 12 mm. in width 

 at the mid-zone. In the same plane the width from the right outer pore of ambulacrum D 

 across interambulacrum C to the left outer pore of ambulacrum B is 13 mm., this being only 

 slightly more than the width given for the interambulacrum itself. There are two columns of 

 plates throughout the interambulacral area, and, as gathered from the impressions on the mold, 

 there are fourteen plates in each interambulacral column. In each of the two lateral inter- 

 ambulacral areas, A and E, which are partially visible, there are five plates in place, and these 

 from their height also indicate that there would be about fourteen plates in one column. This 

 is a very large number for the genus and also for the family. In Recent Eucidaris tribuloides 

 I have found fourteen plates in a column, but such a number is rarely attained in the Cidaridae. 

 In the dorsal portion of interambulacrum C there are two fragmentary plates (text-fig. 239c) 

 which meet in a median angular suture, as usual in cidarids. As seen at one point dorsally 

 where the plates are preserved, the interambulacral plates bevel strongly over the ambulacrals 

 on the adradial suture (text-fig. 239e). It is from this structure that I refer the species to the 

 genus Miocidaris. The interambulacral plates at the mid-zone measure 2.5 mm. in height 

 by about 6 mm. in width, measuring to the middle of the interambulacral area, and about 

 0.75 mm. in thickness. The measurement of the height and width is taken from the internal 

 mold, as no plate preserved is complete in width, but the height as given is confirmed by the 

 plates which are in place. The most complete plate preserved at the mid-zone in area E meas- 

 ures 4 mm. in width, but its admedian portion is hidden beneath the hard siliceous matrix. 

 Unfortunately the worn plates that exist do not show a complete outline nor surface characters, 



