ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 183 



ing to the number of primary arms that spring out directly 

 from the body. As a general rule, according to Wachsmuth/ 

 " the summit plates increase in proportion to the number of 

 primary arms of a species, in the same manner and on the 

 same principle as the plates of the dorsal side." 



We find then, that in the Palseocrinoidea there is an 

 almost complete correspondence in the skeletal developments 

 on the actinal and abactinal surfaces of the body.^ The centre 

 is occupied in each case by a single plate, surrounded by five 

 others, which are situated interradially. But the first and 

 second radials of the abactinal pole appear to be unrepre- 

 sented on the actinal surface. Peripherally, however, there 

 is again a very close correspondence in the arrangement of 

 the plates on the two surfaces. 



The vault of the Palaocrinoidea appears to be something 

 sui generis, and altogether unrepresented in our more modern 

 forms. According to "Wachsmuth,^ with whom I entirely 

 agree, "it cannot in the remotest degree be homologised with 

 the ventral peristome " of the recent Crinoids. '^ It forms a 

 continuation of the radial and interradial series of the dorsal 

 side, and serves merely as a covering and protection for the 

 organs underneath." There is every reason to believe that 

 these were of essentially the same character as in the recent 

 Crinoids, namely, arabulacral grooves with the radial water- 

 vessels underlying them. The ambulacra of a Comatula or 

 Pentacrinus are continued from the arms on to the disc, 

 where they converge towards the peristome ; while in the 

 P al<Eocrinoidea they enter the cavity of the vault by aper- 

 tures at the base of each arm, and continue their centripetal 

 direction as subtegminal channels along the inner surface of 

 the vault. These channels were first discovered in Actino- 

 crmus by Huxley and Billings,* and are " floored by a 

 series of plates which form an elongated arch under them." 

 Alternating with the upper edges of these plates there are 

 found, in good specimens, two rows of minute quadrangular 

 interlocking plates, longitudinally arranged so as to cover 

 the tubes. Hence, under the vault these canals must have 

 been '^ formed of two rows or pieces below {suhamhulaci'al) 

 and two above {super amhtdacraV), all alternately arranged." 

 By Wachsmuth, as by Meek and Worthen,^ they are 



* Loc. cit., p. 187. 



- See Addenda, No. 3, on p. 204. 



3 Loc. cit., p. 190. 



^ " On the Cystidese of the Lower Silurian Rocks of Canada," ' Geo- 

 graphical Survey of Canada,' decade iii, p. 27. 



^ " Notes on some points in the Structure and Habits of the Palaeozoic 

 Crinoidea," 'The Canadian Naturalist,' 1869, pp. M3, Ui. 



