56 ROBERT TRACY JACKSON ON ECHINI. 



in Centrechinus (text-fig. 94, p. 107), Astropyga (text-fig. 99), Echinus (text-fig. 115), and Phor- 

 mosoma (text-fig. 170). Alexander Agassiz (1874, p. 642) said that the compound ainbulacral 

 plates are formed by the splitting of the original plates, but he was clearly mistaken. Passing 

 dorsally, we may pass from compound plates into an area of simple plates which have dropped 

 all attempt at fusion, as seen in Hemi('idaris of the Jurassic (p. 17). Or, as A. Agassiz (1904, 

 Plate 20, figs. 1-4) shows in Salenocidaris miliaris A. Ag., there is a single primary plate ven- 

 trally succeeded by one compound plate, which is again succeeded by simple plates throughout 

 the area, a case of extreme reversion, in which simplicity is the dominant character, and com- 

 poundness or specialization is shown b.y only a single plate. This may be aptly compared to 

 Baculites amongst the ammonoid cephalopods, in which its coiled (ammonite) character 

 is reduced to a minute, almost microscopical basal portion, which is rarely preserved. What- 

 ever the complexity of the ambulacrum in Palaeozoic or later types, it is striking that a simple 

 condition obtains in the nascent plates of the placogenous zone. This all shows the fact of 

 stages in development as evinced by the ambulacrum passing dorsally or ventrally toward the 

 mid-zone, and the importance of differentiating zonal areas in the description of the ambulacrum. 

 While the regular Echini maintain a pair of pores in each ambulacral plate,. or plate ele- 

 menrt in the case of compound plates, the case is different in the irregular post-Palaeozoic Echini. 

 In the clypeastroids below the petaloid area, the ambulacral pores soon drop out, as in Echi- 

 narachnius parma (Plate 8, fig. 4), and the ambulacral plates for the most part at least are non- 

 poriferous. It is true that there are ^'ery fine pores on the ventral side, as described by A. 

 Agassiz (1874, p. 703), but these are so peculiar that they can fairly be distinguished from 

 the usual pores of the ambulacral system. In spatangoids ventrally the pores may be want- 

 ing, or reduced to a single pore, instead of a pair of pores to a plate, in numerous cases. 



As stated, ambulacral plates typically bear a pair of pores representing a single tube- 

 foot, or the pair may be reduced to a single pore, as in some spatangoids. In spatangoids, 

 however, we find a remarkable exception to the rule of a single tube-foot to a plate. In Colly- 

 rites (Plate 3, fig. 15) and other types, as shown by Loven (1874), A. Agassiz (1904), and others, 

 in the basicoronal row, the la, Ila, Illb, lYa, Yb have two pairs of pores, and this is the only 

 case of two pore-pairs, indicating two feet to a simple plate, known in Echini. Loven thought 

 it indicated a compound plate because of the two pairs of pores, or in cases two separate single 

 pores, but always representing two tube-feet. The fact that no suture has been seen in these 

 plates in any of this group of Echini militates against this view, and it seems best to consider 

 this as a peculiar case of two feet to a. simple plate rather than a compound plate, the suture 

 of which has not been observed. As discussed under consideration of these plates (pp. 69, 71), 

 the ambulacral plates in the basicoronal row of spatangoids and clypeastroids may reason- 

 ably be homologized with the first row of ambulacral plates found around the mouth in regular 

 Echini and transferred to the basal row of the corona in the process of evolution of these spe- 



