708 



PKOF. W. J. SOLLAS ON SILUKIAN 



[Nov. 1899, 



latter elements may be, as Duncan supposed, merely accidental. 

 The question, which is of interest, owing to its connexion with that 

 of the origin of the pyramids, cannot be regarded as decisively 

 settled. 



Although teeth have not been observed, the alveoli, as seen from 

 the front, that is, towards the axis of the test, present a deep groove 

 similar to that which occurs in recent Echinoids, and which in 

 them is occupied by the tooth. 



Although badly preserved, partly in consequence of the thinness 

 of the plates, the ambulacra still display characters of interest: 

 a row of minute tubercles borders them on the outer margin, and 

 a double row of even smaller tubercles runs along each side of the 

 middle line (fig. 12). The pore-pairs each lie in an oval depression 



Pig. 12. — Part of ctn amhulacrum of 

 Echinocystis pomum. 



Fig. 13.— Part of the 

 same, more highly 

 magnified. 



(peripodium), and those of the inner series are associated with 

 the larger outer tubercles ; those of the outer row are similarly 

 associated with the inner tubercles (fig. 13). A considerable 

 amount of irregularity disturbs this arrangement, attributable in 

 some degree to imperfect preservation. It is probable, almost 

 certain, that these ambulacral plates, so singularly Echinoid in 

 character, are those of the outer or Echinoid series ; the specimens 

 in our collections are all, so far as I am aware, casts of the external 

 surface only, and the exposed casts of the plates are consequently 

 also superficial. The question then arises whether a second or 

 internal set was also present. It is by no means impossible : in a 

 single example in the Oxford University Museum a minute portion 

 of the test is imperfectly preserved in carbonate of lime ; sections 

 made through this passed through one side of the ambulacrum, and 

 exhibited the appearance shown in fig. 14. Had this structure 

 continued throughout, no doubt could have been entertained on 

 the point, and the existence of a second set of plates would have 

 been proved ; after twenty sections had been obtained in a space 

 of 2 mm., the upper portion disappeared, possibly owing to the 



