222 R. T. JACKSON — STUDIES OF PAL.EECHINOIDEA. 



traced equally well, following the same laws of growth, in this genus^ 

 which we may take in regard to this feature as representing the typical 

 forms of the family. In the allied family, the Lepidocentrida?, as repre- 

 sented by Lepidocentrus and Lepidechinus, the development is so accelerated 

 and the individual plates are so peculiar in form that in details of plate 

 contour and arrangement they make a law unto themselves. 



NOTES ON XENOCIBARIS. 



The genus Xenocidaris, Schultze, is known only from dissociated spines. 

 X. davigera, Schultze (38), the type of the genus, is from the Devonian 

 of Gerolstein, Eifel, This type material, consisting of some 30 spines, is 

 in the L. Schultze collection, in the Museum of Comparative Zoology 

 (catalogue number 3044). Besides the types, a few other specimens of 

 this species are in the same collection. 



The distal ends of the spines are extensively enlarged. They are 

 truncated, the apex being elevated, flat or reentrant ; it is also spinuloses 

 showing much variation in difl'erent spines. The sides of the spines near 

 the distal end are plicated, but in various degrees in different spines. 

 The commoner variations of this protean type are shown well in Schultze's 

 figures. 



The affinities of Xenocidaris are questionable, as it is known only from 

 such limited material. Provisionallv it may be retained in the Archaso- 

 cidaridse, as that is the only family of Paleozoic Echini characterized by 

 large and ornate spines. 



Studies of the T.EPiDocENTRiDiE. 



GENERAL CHARACTERISTICS. 



The Lepidocentridicis a family of Echini associated with the Archa?o- 

 cidarida3 and characterized by possessing strongly imbricating plates and 

 a highly accelerated development in regard to the rapid rate of introduc- 

 tion of interambulacral columns. In this feature it is the most acceler- 

 ated of any Paleozoic Echini. The peristome is small and the single 

 initial plate of the interambulacrum is retained in the adult (Lejn- 

 dechinus), 



Dr Duncan in his " Revision " (9) includes the genera of the Lepido- 

 centridse in the Archseocidaridae, but this seems a mistake, as they are a 

 very distinct and concrete group of forms, and are separated by both 

 Loven (25) and Zittel (44). 



STRUCTURE AND PLATE ARRANGEMENT OF LEPIDOCENTRUS. 



The genus Lepidocentrus^ Miiller, has not been described from this 

 country, though it is quite close to Lepidechinus. In the Museum of 

 Comparative Zoology is a specimen (catalogue number 3040) of Lepido- 



