THK AMHI'LACRUM OF THE CORONA. 57 



cialized forms. In spatangoids, tube-feet as locomotive organs are largely abandoned, and they 

 have only or mainly a respiratory funrtion; here no compound plates are known. With a 

 loss of the locomotive importance the ])lates below the petaloid areas, as in Micraster (text- 

 fig. 6), are relatively high, even hexagonal, as in the embryo and the ancient Bothriocidaris. 

 This primitive character of plates may exist in only one area. In Metalia pedoralis (text-fig. 8), 

 for example, the plates at the mid-zone of the bivium and posterior pair of the trivium are low 

 and wide, with pore-pairs horizontal. In the odd anterior ambulacrum, however, the plates 

 are as high as wide, pore-pairs are superposed and in the middle of the plate, as in Bothrio- 

 cidaris. In such plates we have often in the shape and height, also the superposed position of 

 the pores in each pair, a condition which is a closer approach to the embryonic character and 

 also to the character of Bothriocidaris than is seen at the mid-zone in the adults of any 

 regular Echini excepting the ancient Bothriocidaris. 



The position and mutual relation of the ambulacral pores is of considerable interest. It 

 appears to be typical of the primitive and great majority of adult Echini to have two pores for 

 each tube-foot, and the pair of pores to be surrounded by a more or less strongly marked peri- 

 podium. In many spatangoids the peripodium is absent. In all Palaeozoic genera there are 

 two pores to every ambulacral plate and a peripodium is present apparently without exception 

 (Plate 23, fig. 1). Of course it is such a slight eminence that it is very commonly worn off, 

 but it has been found so widely that its frequent absence is attributed to conditions of erosion. 

 In all Palaeozoic types and most modern Echini, the pores lie either in the middle of the ambu- 

 lacral plate, which is rare, or nearer the next adjacent interambulacrum than the middle of 

 the plate, which is the usual position. In no Palaeozoic type do they lie nearer the median 

 suture of the ambulacrum than the middle of the ambulacral plate. In the demi-plates of 

 Echinosoma hispidum (A. Agassiz, 1904, Plate 48) the pores lie nearer to the mid-suture than 

 to the middle of the plates in which they occur. The same is true of occasional spatangoids, 

 but is exceptional. In Bothriocidaris (Plate 1, fig. 1) the ambulacral pores of each pair lie in 

 the middle of the plate, superposed doi-so-ventrally, and surrounded by an oval perijjodium, 

 the axis of which coincides with the vertical axis of the area. In young Goniocidaris (Plate 2, 

 fig. 2) the pores of each pair also lie superposed as in Bothriocidaris. From the superposed 

 position, which may be called jirimitive, the position of the pores in all Palaeozoic types except 

 Bothriocidaris and in most other regular Echini takes on a change in which there is a revolu- 

 tion through more or less of an angle of 90 degrees from the perpendicular, the revolution being 

 in such a plane that the upper pore of the two is revolved toward the next adjacent interam- 

 bulacrum. The revolution is therefore to the left of the perpendicular in the left half ambu- 

 lacrum and to the right of the perpendicular in the right half-ambulacrum. Only rarely in 

 regular Echini does the revolution exceed 90 degrees from the vertical, and in no case that I 

 have found does the revolution take place toward the middle of the ambulacrum, but in the 



