THE AMBULACRUM OF THE CORONA. 55 



component elements of a compound plate are frequently low, but the plate as a whole is usually 

 high. This method occurs in the Centrechinoida and is seen in some of the Holectypina, but 

 is unknown in the Palaeozoic. A good example is Strongylocentrohis drobachiensis (text-fig. 5a), 

 which at the mid-zone is characterized by compound plates composed of five or six component 

 parts, each of which bears a pair of pores. As a very rare aberrant variant, pores may be 

 wanting in such plates. In three specimens of Strongylocentrotus drobachiensis, collected in 

 Maine, the plates are compound, as usual, but in one area of each specimen the plates dorsally 

 are somewhat distorted and have no ambulacral pores. 



It must be strongly urged that the full character of the ambulacrum in a given type is 

 to be based on the plates at the mid-zone. Below this point, or ventrally, they may not have 

 attained the full differential characters, and dorsal to the mid-zone, one gets into the area of 

 locaUzed stages in development, where the young plates again have not taken on the full char- 

 acter; or again, one may also dorsally reach an area of senescent stages where the full characters 

 are dying out. To illustrate this important principle, in Maccoya burUngloncnsis (Plate 33, 

 figs. 1, 2) the plates at the ventral border are all primaries and pore-pairs uniserial; at the mid- 

 zone the plates are alternately primaries and occluded, and the pore-pairs biserial; dorsally, 

 the young plates are again all primaries and pore-pairs uniserial. In Lovenechinus missourien- 

 sis (Plate 42, figs. 1-4) the plates ventrally are all primaries, at the mid-zone demi- and occluded, 

 and dorsally again primaries. In Melonechinus multiporus (Plate 56, figs. 2-7; Plate 57; text- 

 fig. 245) ventrally, plates are demi- and occluded; at the mid-zone demi-, occluded, and many 

 isolated; close to the ocular, primaries only. In all of these series, and many others as well, 

 the same system exists of simpler conditions ventrally in the plates built when the animal was 

 young, and also in the nascent plates dorsally ; and from there passing to the plates of the mid- 

 zone, it is easily seen how the complex conditions there existent are built up. From these plates 

 also one can arrive at genealogical relations, it is believed, of a very definite character (text-fig. 

 237, p. 231), as discussed later. 



Of the Centrechinoida, in the young Strongylocentrotus, Loven (1892) showed that the 

 plates are simple with a single pore-pair (Plate 3, fig. 11). These plates in the adult are re- 

 moved by resorption, but still the plates at the ventral border are unlike those of the mid-zone. 

 At the dorsal border in Strongylocentrotus, as Loven (1874) showed, and as Duncan (1885) 

 showed in a number of types, the plates are simple, not compound. Duncan says (1885, p. 

 421), "It is an interesting and highly suggestive truth that all the regular Echinoidea should 

 have their most radially situated plates in the form of the simple primaries of the Cidaridae." 

 Loven showed in Strongylocentrotus drdbachiensis that these simple plates are shoved down- 

 ward and become fused to form the compound plates, characteristic of the species. At this 

 region of young plates we find, therefore, as a localized stage, a simplicity like that of less spe- 

 cialized forms of Echini, as (Uidaris. This character of dorsal simple plates is shown very well 



