53 



ambitus, triserially on the actinal side, and the buccal membrane is as in other 

 Diadeniatids. The apical system is unknown, but as it was very large (its diameter 

 about half that of the test) there can scarcely be any doubt that its structure was 

 diadcmatoid It is then difticult to see how Pedinothiiria can prove the Echino- 

 thurids to be an offshoot from the Pedinids, as it has evidently nothing to do with 

 either of these families but must be referred to the Pseiidodiadematidœ (or Diade- 

 matidœ). Gregory has thus not produced any fact extinguishing the fundamental 

 difference between the Echinothurids and the other regular Echinids, viz. the con- 

 tinuation on the peristome of the ambulacral plates. 



Also Neum.wer ') is inclined to think the Echinothurids have been derived 

 from the Pedinids. It must be taken for granted, he argues, that the complicate 

 structures of ambulacra are developed on account of the compression of the plates, 

 „wenn die überzähligen Ambulacraltäfelchen nicht oder nur in ungenügender Zahl 

 auf das Periston! übertreten können. Demgemäss kann auch die Herausbildung 

 von complicierten Ambulacralbildungen nur bei Formen mit starrem Gehäuse statt- 

 linden". Accordingly the ancestors of the Echinothurids must have had a hard 

 test. Among the Palæechinids the genus Palæechinus has a hard test and a begin- 

 ning of the complicate ambulacral structures. It is, however, difficult to understand 

 how the Echinothurids could have developed from a form as Palæechinus, and no 

 intermediate forms are known. It seems then necessary to derive them from the 

 Diadematids, and here Pelanechiniis is thought to be intermediate between Hemipedina 

 and the FLchinothurids. — Pelanechinus, however, has now been proved by Groom 

 to be a true Ecliinotburid, even a very specialized form, and thus it can prove 

 nothing of the ancestry of the Echinothurids any more than Pedinotburia can. 



On a priori grounds it seems, indeed, rather absurd to derive the Echinothurids 

 from the most specialized group of Diadematids, ihe Pedinidce. It is the most natural 

 thing to look out for the ancestors of the Echinothurids among the Palæechinids, 

 and perhaps we may find them among the Lepidocentridœ. The genus Lcpidcchimis 

 has been shown by Jackson'^) to have the ambulacra, but not the interambulacra, 

 continuing over the peristome, and the same probably will hold good for the other 

 genera. The ambulacra are simple, consisting of two series of plates, whereas the 

 interambulacra consist of several rows of imbricating plates. There is thus, in fact, 

 only this one essential difference between Lepidechiniis and the Echinothurids, viz. 

 the structure of the interambulacra, which also obtains between Archœocidaridœ 

 and Cidaridœ. I think then, it will be natural to unite the Lepidocentridœ and the 

 EchinothuridcB into one suborder: Streptosomata. 



The character of the multiple series of plates in the interambulacra (and 

 ambulacra) seems thus to be less important than has been hitherto commonly thought. 



') Stämme des Thierreichs I. 188S). — Cher Palæechinus. Typhlechinus und die Echinothuriden. 

 N. Jahrb. f. Mineral. Geol. u. Pal. 1890. I. p. 84. 



') Studies of Palæechinoidea. Bull Geol. See. America. Vll. 1896. 



