STRUCTURE OF ECHIXOIUS. 175 



considei-ably to the front of the centre. This results in a corre- 

 S2:)onding length or shortness of the part of the ambulacrum 

 within the fasciole, although the number of plates present is the 

 same in both cases. In the shorter areas, the degree of crushing, 

 as indicated by the development of demi- and occluded plates, is 

 relative!}^ increased. This result is obviously brought about by 

 the greater vertical compression in the area. 



For the originals of text-figs. 39 and 4U I have selected speci- 

 mens of almost exactly the same size. There are the same 

 number of ambulaci'als in both in the part of the area considered, 

 but their charactei'S show the two exti-emes of difference. (Both 

 figures are drawn to the same scale directly from photographs 

 in an enlarging lantern.) Text fig. 39 represents a laterally- 

 expanded area, in which sections i. and ii. occupy most of the 

 length up to the fasciole. Text-fig. 40 represents a comparatively 

 narrow area, in which section iii. occupies almost a quartei' of the 

 region within the fasciole. It will be noticed that the degree of 

 plate-crushing is far greater in text-fig. 40 than in text-tig. 39, 

 more notably in the unexpanded adradial than in the perradial 

 tracts. 



IV. The meaning of the Structure. 



The only other Spatangoid in which I have seen a similar 

 structure to that just described is Heterasier ohlongus from the 

 Lower Cretaceous. In that form the pores of ambulacrum III. 

 are more or less biserially arranged, and "plate-crushing," never 

 approaching in intensity that of E. cordatum., is developed. I 

 have been able to satisfy myself that no disturbance of the serial 

 arrangment of the pores, and no trace of " plate-crushing," occur 

 in E chinocardiuvi fiavescens. The same remark applies to the 

 species of Micraster and Hemiaster thsut I have seen, but there are 

 indications of some irregularity in the anterior ambulacrum of a 

 Schizaster from the London Clay in my collection. 



Now in Heteraster, Schizaster, and Echinocardium cordatum, 

 ambulacrum III. is situated in a fairly deep groove ; while in 

 E. Jiavescens, Micraster, and Hemiaster the area is more or less 

 flush with the surrounding test-siu'face in the adapical part. This 

 seems to indicate some connection between the presence of an 

 anterior sulcus and "plate-crushing" in the anterior ambulacrum. 

 I recently (P. Z. 8. 1912, p. 464) argued that the sulcus below 

 the anus in many Jurassic Nucleolitid^^e might be a,scribed to an 

 excessive growth of the plates there as a result of the interference 

 of the pei'iproct. The same explanation seems to suflice in this 

 case. The number of interambulacral plates bordering on ambu- 

 lacrum III. in E. cordatum remains constant during all the later 

 growth-stages, but the number of ambulacrals increases steadily 

 with the growing size of the individual. This indicates a propor- 

 tionate increase in the potential length and expanse of the 

 ambulacrum compared with that of the surrounding part of the 

 test. That would result in a sagging inwai-ds of the over- 



