GEFEBA AND GROUPS OF THE ECHINOIDEA. 17 



this anomaly, observe that the direction of the overlap is tlie 

 same as that seen in Echinothuridse. Now there is no doubt 

 that this mistake about the direction of the overlap of the ambu- 

 lacral and interradial plates in Echinothuridse originated with 

 the late Sii* Wyville Thomson, who wrote in ' The Depths of the 

 Sea,' 1873, p. 158, that the overlap was of " the plates of the 

 interarabulacral arese from the apical pole towards the mouth, 

 those of the ambulacral arese from the mouth towards the apical 

 diisk." He referred, for the purpose of illustration, to his figure 

 28 (on p. 157). Unfortunately this figure is of part of the inside 

 of the test of Calveria (Asthenosoma) hy stria;, Wy. Th. The 

 direction of overlap is always considered in relation to the out- 

 side of tests. In 1874 Mr. E. Etheridge, Junr. (Quart. Journ. 

 Geol. Soc. vol. XXX. p. 307), quoted Sir "Wyville Thomson's words, 

 and it became generally believed that the overlap of the plates 

 of the Echinothuridse was in the opposite direction to that of the 

 Perischoechiijoidea. Bat Sir Wyville Thomson also wrote in 

 1874 (Phil. Trans. Eoy. Soc. p. 730), after the publication of Mr. 

 Etheridge's jDaper, as follows : — " The plates of the interambulacral 

 areas overlap from the mouth towards the apex, and the am- 

 bulacral plates in the opposite direction." A. Agassiz also 

 pointed out that the overlap is normal. Again, the most striking 

 character of Worthen and Miller's genus is the projection in the 

 form of humps of the ambulacra near the apex. But that con- 

 dition is so anomalous that one is tempted, especially after the 

 explanation of the abnormal overlap, to believe that the reverse 

 condition should be seen from outside, and that the humps are 

 really marsupia seen from within the test. The genus as defined 

 by its authors is given, but the remarks just made should have 

 their weight. 



Genus HTBOCHiisrirs, Worthen Sf Miller, 1883, Geol. Sf Pal. of 

 Illinois, vol. vii. p. 331. 



Test flexible, subspheroidal, consisting of 5 ambulacral and 

 the same number of interradial areas ; protuberances at the apical 

 ends of the ambulacra {possihly marsupial cavities^. 



Ambulacra composed of numerous (10) ranges of alternating 

 and overlapping plates, and even more at the ambitus ; each plate 

 perforated in the central part by a single pair of pores. The 

 plates imbricate from below upwards {prohally from alove 

 downwards). 



LINN. JOUKN. — ZOOLOGY, VOL. XXIII. 2 



