Dr. F. A. Bather on Piotoeehinus, Austin. 45 



largely due to the imbrication of the ambulacrals. If the 

 same plates were viewed from the outside the pores would 

 in any case not be so near the edge ; but, further than this, 

 the pore-canals slope slightly from inside to outside in an 

 adapical direction, so that they would have opened on the 

 on Kith- still nearer the middle transverse line of the plate. 

 The pore-pair opens on the inside at the bottom of a slight 

 depression, one side of which forms part of the transverse 

 ridge. The long axes of these internal pcripodia lie at an 

 angle to the perradius, the inner pore being more adoral in 

 position than the outer. The adradial end of the peripodial 

 rim is depressed, presumably for the passage of the side- 

 branch from the radial water-vessel which passed to eacli 

 pore-pair behind the transverse ridge of the adjoining orad 

 amhulacral. In the case of the two or three ambulacrals 

 nearest the peristome this passage is quite arched over by 

 the above-described processes; and it looks as though the 

 radial vessel were also covered by these processes when they 

 were complete. This latter arrangement increases the re- 

 semblance of the processes to auricles, and suggests that in 

 serial sections obtained by grinding down such a fossil 

 embedded in matrix the processes, especially if broken, might 

 possibly be interpreted as floor-plates. 



A few Radicles are preserved on both faces of the speci- 

 men. The evidence that they belonged to this individual is 

 inconclusive, but they are of a character consonant with that 

 view. The one shown just above the numeral 4 in the text- 

 figure is probably a primary radiole. It is 4*3 mm. long, has 

 a slightly enlarged base, and is almost smooth, with traces 

 of longitudinal fasciculate micro-structure. It closely re- 

 sembles, except in its smaller size, the primary radiole of 

 Pholidocidaris irregularis (Jackson, 1912, pi. 75. fig. 5). 

 The other fragments are rather thinner, but show the longi- 

 tudinal striation more plainly ; they belong probably to 

 secondary radioles. 



The minute Structure of the Stereom is visible under a 

 strong lens on several of the plates. In the ambulacrals it 

 is rather coarser and quite irregular. In the interambu- 

 laerals it is finer, with the meshes arranged in regular, 

 though not necessarily straight, rows. 



Systematic Position. 



Following the Classification of R. T. Jackson (1912), and 

 using his Key (pp. 201-208), we note that the number of 

 columns of ambulacrals and interambulacrals, their strong 



