224 R. T. JACKSON — STUDIES OF PAL^ECHINOIDEA. 



be compared with the similar irregular position of the seventh column 

 in 3Ielonites septenariiis (plate 9, figure 49). It is noticeable in Lepidocen- 

 trus mulleri that the initial plates of newly added columns do not show 

 the pentagonal form which is shown to be characteristic of most Paleo- 

 zoic Echini, and the same remark applies more or less closely to Lepi- 

 dechinus (plate 7, figure 42) and Pholidocldaris (plate 9, figure 54). 



In the large number of columns of interambulacral plates attainedr 

 this species is one of the most extreme types of all echinoids, a similar 

 condition being seen in Lepidechinusrarlspinus,}iii\], and Melonites gig an- 

 iens Jackson, (plate 5, figure 21), both of which have 11 columns (see sys- 

 tematic table facing page 242). 



No genital or other j^lates indicating the dorsal or ventral area being ex- 

 istent, this specimen of Leplducentrus mulleri has to be oriented by the 

 direction of introduction of columns (compare with Melonites^ plate 2, 

 figure 2, and Lepldechinus, plate 7, figure 42). This being done, it is seen 

 (figure 2) that the interambulacral plates are strongly imbricated adorally 

 and laterally ; the median plates overlapping those on either side and ad- 

 ambulacrals overlapping adjacent ambulacrals. Accepting this method 

 of orienting axes as correct, and it seems to be clearly proved b}^ the numer- 

 ous types studied, it follows as a consequence that Schultze's (38) original 

 figure of this specimen was incorrectly oriented. Miiller's (35) figure of 

 Lepidocenlrus rhenanus, Beyrich, was also incorrectly oriented, as ]n'oved 

 positively in this case by the presence of jaws ; and Professor Zittel, in his 

 recently published " Grundziige der Pala3ontologie," has copied the figure 

 and corrected the orientation. 



The two species, and perhaps the two specimens mentioned, are the only 

 ones in the genus in which a comparatively complete test is known, and 

 this incorrect orientation in the figures cited has apparently led to a mis- 

 take in regard to the direction of iml^rication of the plates. We find the 

 statement by Dr Duncan (9) and others that the interambulacral plates of 

 Lepidocenlrus imbricate aborally and laterally, whereas it should be cor- 

 rected to read adorally and laterally. The same authority (9) describes 

 the interambulacral plates as hexagonal. In Lepldocentrns rhenanus, Beyr. , 

 they are nearl}^ or quite hexagonal, with the corners rounded, as figured by 

 Miiller ; but in L. mulleri (figure 2) they are more nearly rhombic in form 

 and none are strictly hexagonal ; in L. eifelianus they are scale-like, with 

 rounded rather than hexagonal sides. I feel it necessary to make a point 

 of correcting this statement because the form of the plates in this genus 

 is considered as an aid in the systematic classification of the three species. 



In the study of tlie plates of Lepidocentrus mulleri help has been obtained 

 from the large number of dissociated interambulacral plates in the 

 Schultze collection. These plates are the types of Schultze's original 



