52 _ 



Echinothiiridæ as a suborder Streptosomata of the Diadematoidea against all the 

 other families as another suborder Stereosoniata (Stereodermata) '). Pomel goes even 

 further, as he unites the Echinothiiridæ with the Cidnridœ as Holostomata against 

 all other regular tLchinoids (the Palechinoids excepted) as Glyphostomata. But it is 

 to be remembered that he did nol know the occurrence of gills in the Echino- 

 thurids. If he had known that (and he might have known it, as Ludwig's paper 

 on Asthenosoma uarium was published in 18S0, Pomel's work in 1883), he would 

 certainly have altered his classification. Gregory goes the other way, making the 

 Echinothurids only a family of his suborder Diademina. He finds the characters 

 given for the suborder Streptosomata indistinctive. „In the Pedinid Astropgga and 

 the Diademid Helikodiadema the test is more or less flexible. Both external and 

 internal branchiae arc as well developed in Diadema as they are in Phormosoma. 

 In those of the Stereosomata which have plates on tiie peristomal membrane these 

 are always ambulacral" -). It is very curious that Gregory has been induced to 

 make the latter assertion, which is as much erroneous as it is fundamental. In 

 all the regular Echinids, except the Cidarids and F]chinothurids, there are only five 

 pairs of buccal ambulacral plates, provided with tube feet, which are mostly larger 

 and a little different from the other tube feet (in a single instance, Pleiirechimis 

 Döderleini there are constantly only five unpaired buccal plates and tubefeet; see 

 the description of this species below). Besides these buccal plates there may be 

 more or less numerous plates in the buccal membrane, but these are always 

 irregularly placed, and have nothing at all to do with either the ambulacral 

 or interambulacral coronal plates; they are formed in the buccal membrane 

 itself, the youngest ones being found nearest to the edge of the peristome, not 

 detached from the lest, and there are never found tubefeet on these plates. In 

 the Echinothurids the plates covering the peristome are true ambulacral plates, all 

 bearing tubefeet, and the first pair of tubefeet are not larger than or different from 

 the following ones. In the Cidarids the same fact holds good, only here also the 

 interambulacral plates continue over the peristome. — These fundamental facts 

 have already been clearly pointed out by Ludwig^), who has, indeed, first placed 

 the Echinothurids as a separate group against all the other regular Echinids above 

 the Cidarids. — Gregory further thinks he has found in the genus Pedinothuria 

 a form intermediate between the Pedinids and the Echinothurids, showing the 

 latter family to be „an offshoot from the Pedinidæ" (Treatise on Zool. p. 310). This 

 genus has perforate, crenulate tubercles, the pores are arranged biserially at the 



') W. Kekping. Notes on the Palæozoic Echini. Quart. J. Gcol. Soc. XXXll. 1876. p. 40. Under the 

 section Stereodermata Kkkping includes both regular and irregular Echini (Endocyclica and Exocyclica 

 Wright). The name can then scarcely be used in the meaning of Duncan and Lambert. 



') On the affinities of the Echinothuridæ; and on Pcdinotliuria and Helikodiadema, two new 

 genera of Echinoidea. Quart. Journ. Geol. Soc. LUI. 1897. 



') Über Asthenosoma varium Grube und über ein neues Organ bei den Cidariden. Morph. Studien. 

 II. p. 17. (1880.) 



