REPORT ON THE ECHINOIDEA. 77 



the gills do not pass through cuts in the test so prominent in the Diadematidae, but force 

 their way through the membranous space between the coronal plates and the actiual 

 membrane proper at the angle of the poriferous zone and interambulacral area. 



The discovery of recent Echinothurid^ has naturally led to a renewed discussion of 

 their affinities with the Palseechinidie and other Palaeozoic Echinids, and more particularly 

 of the systematic relations of the Palteechinidfe to the Desmosticha. The relationship to 

 the Cidaridse and Echinothuridge is certainly not very remote, as will be seen when we 

 come to compare the apical and actinal systems of the Cidaridae with those of the 

 Palaeechinidse, and the structure of the primary tubercles of the actinal surface of the 

 recent Echinothuridse to the primary actinal j)lates oi ArclicBOcidaris and Eocidaris. 



As far back as 1857 Mtiller^ called attention to the imbricating plates of the test, 

 not only of Lepidocentrus but also of Archceocidaris, and both Loven^ and myself 

 called attention to the imbrication of the coronal plates of the Perischoechinidse as a 

 general character of the gi-oup. In 1874' I called attention to the fact that Mtlller's 

 observation had escaped the attention of the American Palaeontologists, to whom we owe 

 the description of so many of the genera of this interesting group of Echinids. Later 

 English writers'* on the subject, who have, in consequence of the discovery of the recent 

 genera Asthenosoma and Phormoso'nui with the imbricating coronal plates, taken up the 

 question again seem likewise to have completely overlooked what Mliller had published 

 on the subject. 



There are in the collection of the Museum of Comparative Zoology a number of 

 specimens of Palaeechinidae, which with the fine collection of Palajechinidaj from the Bur- 

 lington Umestone made by Mr Wachsmuth, which he has been kind enough to lend me 

 for inspection, has enabled me to examine the greater number of the genera thus far 

 described, and to satisfy myself, from personal examination, of the structure of the 

 coronal plates of the Perischoechinidae. 



As has been pointed out by Etheridge '' there are two very distinct types among the 

 Palaeechinidae. Those in which, as in Lepidesthes, Lepidechinus, Echinocijstites, Lepidocen- 

 trus, and the like, we have comparatively thin coronal plates imbricating like the tiles of a 

 roof both towards the actinal and abactinal region as well as laterally, and those in which, 

 as in Oligoporus, Pcdceechiims, and Melonites, the coronal plates are of great thickness and 

 on which the plates abut by more or less bevelled edges but still retain the same lateral 

 and vertical (actinal or abactinal) direction. This division, of course, depends entirely 

 upon the thickness of the plates of the test, and is not based upon important structural 

 features, though the facies of such genera as Oligoporus and Lepidesthes would at the first 



» J. Mliller, Ueber neue Echinoderinen des Eifeler Kalkes, Berlin, 1857 ; Abhand, d. Berlin Akad. fiir 1856. 



* S. Loven, Etudes sur les Echinold^es . 



^ Revision of the Echini, part 4. 



« J. Young, Geological Magazine, vol. x. p. 301, 1873 ; W. Keeping, Quar. Jour. Geol. Soc, vol. xxxii. p. 35, 1876. 



^ R. Etheridge, Jr., Quar. Jour. Geol. Soc, 1874, vol. xxx. p. 307. 



