332 ROBERT TRACY JACKSON ON ECHINI. 



"The ambulacra are composed of four columns of plates. The inner rows meet in a zigzag 

 radial suture. The outer end of each plate, bearing a pore-pair, interlocks with the inner ends 

 of the two adjacent outer plates. The outer plates then expand and bear pore-pairs alternating 

 with those of the inner series. This structure obtains in all visible parts of the ambulacra, 

 from the apical end to the ambitus. The number of ambulacrals corresponding with an inter- 

 ambulacral is 10 to 12, and this relation also appears fairly constant, the ambulacrals being re- 

 duced in height in proportion to the height of the interambulacrals. The fifth interambulacral 

 from the summit, in the left anterior interambulacrum, has a height of 4.5 mm. and a great- 

 est width of 4.7 mm. The total width of the anterior ambulacrum at the adoral margin of 

 this plate is 6.2 mm. ; about five and a half ambulacrals of the outer series abut against it, so 

 that the height of each ambulacral here is about 0.8 mm. 



"The whole of the poriferous tract of each ambulacrum is depressed, and from it the 

 perradial tract of the ambulacrum rises in a well marked curve. The perradial tract bears 

 miliaries like those of the interambulacrals, from 6 to 8 being on the adradial portion of each 

 ambulacral plate of the inner series, and a single miliary on the adradial tongue of each ambula- 

 cral of the outer series. These miliaries are regularly arranged in diagonal rows, crossing at a 

 constant angle. I am unable to detect on these miliaries any definite mamelon or any perfora- 

 tion, or any suggestion of a scrobicule surrounded by a scrbiocular ring. That is why I call 

 them miliaries and not tubercles. W. H. Baily has described and figured very precisely all 

 those appearances in the corresponding structures of Palaechinus elegans (Journ. Roy. Geol. 

 Soc. Ireland, vol. 1, p. 63) and P. gigas (op. cit., vol. 4, p. 41), and I should be glad to know 

 whether anyone has ever confirmed his observations. [I have not seen such a structure in 

 any species in this family. R. T. J.] 



"The pore-pairs are surrounded by a rim [peripodium], and the interporal septum is well 

 defined and prominent. These structures, however, have been considerably worn in the prepa- 

 ration of the specimen. The pores are not circular. Those of the inner pair are together like 

 the outline of a hen's egg, cut across at the septum, the broader half of the egg being adradial. 

 In the outer pore-pair, the adradial pore is of similar shape, but the outer pore lies close to the 

 margin of the ambulacrum and appears as though compressed thereby into an ellipse with its 

 long axis parallel to the margin. 



"The adradial suture. Owing to the position of the outer pore-pairs and to the sudden 

 uprise of the adjacent interambulacrals, the ambulacrum, when viewed superficially, seems to 

 dip under the interambulacrum, as in the flexible suture of primitive Cidaridae. The suture 

 was probably not flexible; the abutting plates are very thick, about 3.5 mm. near the ambitus; 

 and the suture-face of the interambulacrum dips towards the perradius [that is, ambulacrals 

 bevel over the adradials, a typical character in this family. R. T. J.]. The pore-canals of 

 the ambulacra also slant in the same direction. These facts, seen somewhat obscurely at the 



