PALEONTOLOGIC CONTRIBUTIONS 35 



9, figure 2). The ambital accessory plates in the outer interradial 

 disk areas between the arms are so strongly developed in Hall's 

 types that they form a very marked weblike structure (not well 

 brought out in Hall's figure) between the arms (see plate 8, figure 

 i). In the material from Saugerties only the largest specimens 

 exhibit a few accessory plates in the interradial spaces which are 

 easily overlooked. 



On the actinal (ambulacral) side of the specimens the differences 

 between Hall's types and the mature Saugerties specimens are not 

 quite so striking. The number of plates is smaller; there are only 

 about thirty-two adambulacrals against thirty-eight to forty in the 

 type of the species and about twenty against twenty-seven to 

 twenty-eight inframarginals. The ambulacral grooves which are 

 slightly petaloid in the gerontic individuals, are straight in the 

 young and mature specimens from Saugerties ; and the difference 

 in size between the adambulacrals and inframarginals is not nearly 

 so large as in Hall's type, for the reason that while the adam- 

 bulacrals are in both of the same size, the inframarginals have 

 grown much broader in the old individuals. On that account they 

 also show more distinctly along the edges of arms on the abactinal 

 side than they do in the Saugerties material, for the supramarginals 

 have not grown correspondingly in width. 



All these differences between the mature specimens of Saugerties 

 and the supposed gerontic type of the species produce such marked 

 differences in general habit that the two could be well considered 

 as at least belonging to different varieties, if not species, were it 

 not for the fact that the still younger stages show that the differ- 

 ences are all of ontogenetic development. As it is, the specimens 

 from Saugerties resemble much more those of Spaniaster, both in 

 the actinal and abactinal aspects, than those of the gerontic 

 Devonaster. 



Besides the many mature individuals of D . eucharis, the 

 material from Saugerties contains also several very young individ- 

 uals, one with a radius of but 4 mm. These furnish some facts 

 which are probably of more than ontogenetic interest. We have 

 already mentioned above the different structure of the abactinal 

 disk and the absence of accessory plates in the abactinal disk and 

 arms. These plates are quite clearly only a later acquisition and 

 are in line with the general tendency of all progressive asterids of 

 the Phanerozonia to get away from the solid caselike primary 

 skeleton. Schuchert (op. cit., p. 32) describes this tendency as 

 follows : " The change lies mainly in the increasing number of the 



