NEW PEATUEES EN" PEEANECHIXTJS COEAELENTTS. 



705 



side of the test immediately after the equator is passed. Their 

 boundaries, before only slightly and regularly undulating, here 

 become distinctly curved, while their inner extremities become much 

 less in width than their outer ; the result is that a triangular 

 depression (fig. 6) is left in the interambuiacral areas, giving the 

 apical area a pentagonal outline. The uppermost plates at the same 

 time overlap from below upwards ; this may, however, be due to pres- 

 sure, but I believe is natural, as a certain amount of bevelling off at 

 the edges is perceptible. 



It is noteworthy that the direction of overlap differs from that of 

 the Echinothuridae, but is the same as in Astropyga *. This genus 

 it also resembles in the behaviour of the interambulacra on the 

 aboral side of the test, the plates undergoing precisely similar changes 

 in shape and disposition f . 



The triangular depressions I believe to be due to the projection 

 of the genital plates, which were probably large ; the pentagonal 

 outline of the central space probably represents the shape of the 

 apical disk, which must have been of considerable size. 



This view is confirmed by the behaviour of the ambulacra, which 

 narrow rapidly on the upper side of the test (fig. 5), and show cha- 

 racters which indicate that they are not far from their apical termina- 

 tion (fig. 4, 6). 



In their elongated shape, general appearance and behaviour, and 

 in their loose connexion, the interambuiacral plates resemble those 

 of the Echinothuridse and Astropyga, the upper plates especially 

 (see figs, of Asthenosoma and Phormosoma J). 



In the notched peristome, and the contrast between the plates of 

 the corona and peristome, and between the ambulacral and interambu- 

 iacral areas, the test resembles that of Echinidse and Diadematidse. 



The Ambulacral Areas (figs. 2 & 3) were not correctly described 

 by Keeping §. The structure of the plates is best shown at the 

 equator. Each plate is pentagonal and has nine pairs of pores dis- 

 posed in three oblique rows, each pair surrounded by a peripodium. 

 It is traversed by two sutures, which extend from margin to margin, 

 and indicate the boundaries of the three primary plates which enter 

 into its composition ; each of these primaries bears a tubercle and 

 has at its outer border a pair of pores. The middle primary is the 

 largest and supports the largest tubercle ; its inner border occupies 

 by far the greater part of the boundary formed by the median 

 ambulacral line ; the aboral primary comes next in size, while the 

 adoral is smallest and bears the smallest tubercle. In addition to 

 the three primaries there are six demiplates, which do not extend 

 from margin to margin ; each bears ordinarily a pair of pores. The 

 grouping of these component plates is seen in fig. 2. The adoral 



* Wyville. Thomson, "On the Echinoidea of the 'Porcupine' Deep-sea 

 Dredging Expedition " (Phil. Trans. Royal Soc. vol. 164, part 2, p. 732). 



t I am indebted to Prof. Jeffrey Bell for an opportunity of examining this 

 and other genera in the British Museum. 



t ' Challenger ' Report ; Echinoidea, Al. Agassiz ; and Wyville Thomson, Phil. 

 Trans. Eoyal Soc. 1874, pi. lxv. 



§ W. Keeping, loc. cit. 



