132 ROBERT TRACY JACKSON. ON 



In Echini, the young has a single plate at the ventral border of each interambula- 

 crum (Strongylocentrotup, Fig. 112). This character is a feature of the archaic Bothri- 

 ocidaris, from the lower Silurian of Russia, as I have pointed out ('96). This single 

 plate in most Echini is lost by the resorption of the ventral border of the corona through 

 the encroachment of the peristome. The primitive character of the single plate is 

 retained in many highly specialized types, as Lepidechinus rarlsplnus Hall and Pholl- 

 docidaris meeki Jackson, of the Subcarboniferous, also in spatangoids and clypeastroids. 

 These several types are highly specialized in the sum of their characters, but in the local- 

 ized area aljout the mouth have retained the embryonic and highly primitive character of 

 single plates at the ventral border of the interambulacra. 



In Lepidechinus rarispinus Hall, of the Subcarboniferous, the characteristic plates of 

 the interambulacral areas are rounded and imbricating. New plates in Echini are 

 added on the dorsal border of the corona, and at this area the plates of this species are 

 less imbricate and hexagonal. Close to the area of the genital plates they appear to be 

 rhombic in outline. In Melonites and other less specialized Palaeozoic Echini the 

 plates of the interambulacra are hexagonal, and near the dorsal portion of the area are 

 rhombic in outline. Also the plates of these types are not imbricate (except adam- 

 bulacral plates which extend under adjacent ambulacrals ; see Jackson and Jaggar, 

 '96) . The evidence, then, is that in Lepidechinus rarisjmius the young newly formed 

 plates repeat the characters seen normally in the plates of other less specialized types. 

 This relative simplicity of young plates in Lepidechinus first suggested the idea of local- 

 ized stages in development, and was published essentially as described here (Jackson, 

 '96). Echinoderms are essentially plate-bearing animals. It is probable that in the 

 genesis of newly added plates in many types localized stages may be found which are 

 comparable to stages seen in the young. 



Spondylus. Beecher ('98) has pointed out the fact, that in brachiopods and some 

 mollusks, especially pelecypods and conical non-coiled gastropods, localized stages in 

 development may be seen in the multiplication of surface ornamentations through inter- 

 polation. Many mollusks present stages of spine development in two directions. (1) 

 The normal series is represented by the succession of spines along a single sector of 

 growth. In the radial plications of a Spondylus or Lima, the earliest and primitive spines 

 are found near the beak, while those on the ventral border of an adult specimen are the 

 latest and most highly developed. (2) By the radial divergence of the ribs and by the 

 interpolation of additional ribs at intervals, as many transverse series of compound spines 

 appear on the periphery as there are primary radii. Hence in a given case, there may 

 be two radii continuing to the beak, then by interpolation there are successively 5, 11, 

 2-3, etc., radii, the highest number being found at the periphery (Figs. 12, 13). Mort- 



