VIII. Comparison with Structure of Asterozoa 1915 318 



passage of ampullse." This is probably true, and yet it does not 

 follow that pores were not developed before Devonian time, or even 

 that they were absent in the very cases to which Mr. Spencer refers. 

 The true pores in Xenaster are difficult to see, by reason both of their 

 position and their small size ; they were probably even more difficult 

 to see in the predecessors of Xenaster. Take Lindstromaster, for 

 instance, from the Wenlockian of Gotland. Dr. J". W. Gregory, in 

 his exceedingly succinct account (Geol. Mag., 1899, p. 347), said: 

 "The ambulacral plates are boot-shaped. The pores for the podia 

 are large." Apparently these statements were based on plates in 

 which the depression was still partly filled by matrix. In other 

 regions, however, the matrix has been most carefully removed, and 

 the true structure is represented in Liljevall's accurate drawing 

 (Geol. Mag., 1899, pi. xvi, fig. lb, the ray in the south-east position, 

 on the west and east sides of it respectively). The details are 

 obscured in the half-tone reproduction, but I had the opportunity of 

 comparing the original with the specimen itself in special reference 

 to this point. The median ridges of the floor-plates run diagonally 

 from the proximal adradial corner to the distal outer corner of each 

 plate, so that, if the sutures be not carefully observed, the plates 

 appear to be directed distalwards as they pass outwards. The 

 sutures, however, really pass in the contrary direction. Thus the 

 position where the pore should be looked for is where the suture 

 meets the outer end of the ridge. Here the pore is indicated 

 by Liljevall, almost hidden by the ridge and the overhanging 

 " adambulacral and here it does seem really to occur in the 

 specimen, though proof by sections or grinding down is wanting. 



If pores were absent in all the pre-Devonian Asterozoa, it would 

 be very difficult to understand how the relatively narrow pores 

 of Xenaster were formed. Starting with pressure of an incipient 

 ampulla outside the floor-plates, one would expect to observe a gradual 

 deepening of the excavation until it broke through into the thecal 

 cavity as a relatively wide hole. Such is in fact the appearance 

 presented by Professor Jaekel's drawing of Siluraster perfectus, from 

 the uppermost Ordovician of Bohemia (November, 1903; Zeitschr. 

 deutsch. Geol. Gesell., vol. 55, Protokoll. p. 108 = p. 15). As our 

 knowledge increases it may be that we shall find among the early 

 Asterozoa, as among the Edrioasteroidea, some genera with podial 

 pores, others without, forming parallel lines of descent. The presence 

 of endothecal ampullae is necessarily dependent on the existence in the 

 rays of a thecal cavity large enough to contain them. 



The Cover-plates of JSdr toaster appear to find their homologue in 

 the so-called " Adambulacralia " of the starfish. These elements are 

 thus described by Spencer in Archaster typicus (p. 13) : " They are 

 irregularly pentagonal in shape. The outer side runs almost parallel 

 with the length of the groove. The face nearest to the groove has 

 two facets [i.e. two sides of the pentagon]. The proximal facet is 

 short and straight, the distal facet somewhat longer and slightly 

 concave. We shall see that the appearance thus presented is 

 characteristic of primitive Asterozoa and primitive Ophiuroidea as 

 well as of the Asteroidea." This description applies almost exactly 



