NOTES ON PERISCHODOMUS. 225 



work, and are in the Museum of Comparative Zoology. One set of plates 

 is from the Devonian of Rommersheim, near Prum, Eifel (catalogue 

 number 3041), and another set is from Gerolstein, Eifel (catalogue num- 

 bers 3042 and 3045). The original types of the spines, presumably pri- 

 maries from interambulacral plates, are also in the collection. They are 

 from Gerolstein, Eifel (catalogue number 3043). 



The interambulacral plates in the upper part of the test (figure 2) are 

 all nearly rhombic in outline, and ventrally the plates have the angles of 

 the rhomb truncated by curving additional sides. It has been shown 

 that in Melonites the newly formed or young plates at the dorsal border of 

 the corona are rhombic in outline (plate 3, fi.gure 13, and plate 5, figures 

 20 and 21). A similar condition has been approached in Oligoporus 

 (plate 6, figure 31) and in Palseechinus (plate 7, figure 36). It appears, 

 therefore, that the plates of Lepidocentrus mulleri throughout the life of the 

 individual retain nearly or quite the simple character which is seen in 

 young, newly added plates of unspecialized types. (For further discus- 

 sion see page 228.) 



The interambulacral plates of Lepidocentrus mulleri have a single com- 

 paratively large primary spine boss, occupying various positions on the 

 plate, though none were seen in the center, the usual place for such 

 bosses. Secondary spine bosses are scattered over the surfaces of the 

 plates. This condition of primary and secondary bosses is interesting 

 for comparison with the similar condition shown by Meek and Wor- 

 then (31) in Pholidocidaris irregidaris, another aberrant echinoid, but in a 

 quite distinct family (see table of classification facing page 242). The 

 spines of Lepidocentrus miUleria,Ye small, acicular and swollen at the base. 



In Lepidocentrus eifelianus, Miiller, we have another, but quite closely, 

 allied species, which is known only from dissociated plates. Plates of 

 this species from the Devonian of Nohn, Eifel, are in the Schultze col- 

 lection in the Museum of Comparative Zoology (catalogue number 3068), 

 and spines attributed to the same (catalogue number 3069) ; also a large 

 series of plates from the Devonian of Rommerscheim (catalogue number 

 3061). This species differs from L. mulleri in having much thinner plates, 

 and also the plates are more rounded in form, exceeding in this feature 

 any plates found in that species. It is therefore apparently more spe- 

 cialized in this peculiar matter of plate form. The spines do not differ 

 essentially from those of L. mulleri. 



I 



NOTES ON PERISCHODOMUS. ^ 



The genus Perlschodomus is known only from two species. One, P. illi- 

 nolsensls, Worthen and Miller (42), is imperfectly known. The tyj)e is 

 in the Illinois State Museum at Springfield, Illinois (see foot-note, page 



