1904.] Affinities of Palceodiscus and Aydacrinus. 33 



plates, the inner he compared to the ambulacral ossicles of Asteroids. 

 Thk suggested a solution as bo the long-standing puzzle of the relation- 

 ships yf Asteroids and Echinoids. One specimen of Palaeodiscus showed 

 plates vn the vicinity of the mouth which had fortunately escaped 

 solution. That no doubt might remain as to the existence of the 

 double series of plates, one of the ambulacra of this specimen was cut 

 away and ground. The result of the investigation, together with some. 

 amplifications in the description of the genus, form the first part oi 

 this paper. 



Description. The general appearance of the form of Palseodiscus, as 

 preserved in the shale, suggests that it was originally discoid and 

 rounded. All the known remains are flattened almost completely in 

 the horizontal plane, and are thus quite different from the impressions, 

 of the globular Echinocystis, which lie in almost any plane. (Plate 1,, 

 fig. 1.) Usually the peristomal region is occupied by a cast of the 

 lantern of Aristotle. One of the specimens as described by Professor 

 Sollas possesses, however, an impression of the peristome, which is seen, 

 to be occupied by the plates of the ambulacra. No interambulacral 

 plates are present here. This is the first of the many embryonic and 

 primitive characters which characterise the genus. It recalls the 5 

 arrangement which occurs in certain other families of the Palsechinoidea,. 

 namely, the Bothriocidaroida, the Lepidocentridse and the Lepidesthidse.. 

 Further, Professor Love'n (11, pp. 6, 7, 12) has shown that in the young 

 stages of Goniocidaris and Strongylocentrus the ambulacral areas meet 

 and shut out the interambulacral plates from the mouth region, although 

 later these latter plates invade the peristome. 



The interambulacrum, as shown in fig. 2, begins with a single plate. 

 This acquires great importance from the studies of Loven and Jackson. 

 Love'n (11, p. 12) has shown that in the interambulacrum of young 

 Echinoids there is but a single plate. During later growth the initial 

 plate is gradually resorbed. The initial plate is represented, however,, 

 in the adult clypeastroids and spatangoids as also in the Palaeozoic 

 genera Melonites, Lepidechinus, and Pholidocidaris. There is one 

 genus of Palaeozoic Echinoid Bothriocidaris which possesses only one 

 row of plates in the interambulacral regions. This has led Jackson 

 (9, p. 233) to suggest that Bothriocidaris represents an ancestral form 

 from which the remaining Echinoids may be derived. The form of the- 

 inter-ambulacral plates of Palseodiscus as detailed below shows, however,, 

 that even if we take the criterion of the interambulacrum, it appears- 

 much more primitive and ancestral than that of Bothriocidaris. The 

 majority of the interambulacral plates are rhomboidal in form, and it is> 

 only near the peristome that they become polygonal. Love'n (11, p. 20) 

 shows that the newer interambulacral plates arise distally from the 

 mouth, and are nearly quadrangular. Later they become polygonal 

 owing to growth pressure. Jackson (9, pp. 225, 228, etc.) in his studies 



VOL. LXXIV. D 



