142 EGBERT TRACY JACKSON ON 



may be traced wliicli, in general, can be correlated with a similar series in young develop- 

 ing individuals. 



In the Palaeozoic echinoid Lepidechiniis the new plates added at the dorsal border of 

 the corona are ditferent from the older plates in the same specimen, and present features 

 similar to those seen at the same area in allied less specialized types. 



Localized stages in development as represented in budding animals and echinoderm 

 plates, are seen only in the youth of the bud or jtlate, and during growth are wiped 

 out by changes in the growing part. In the following cases of ornaments of mollusks 

 and septa of cephalopods a different condition exists. The localized stages are perma- 

 nently fixed in the part and are not capable of modification during further growth of the 

 animal. 



Beecher (98) has pointed out that the spines of radial plications which originate by 

 inter20olation in certain brachiopods and mollusks, repeat in their inception and subsequent 

 growth the characters seen in early stages of older plications which originated earlier in 

 the life of the individual; Spondylus, Figs. 12-13, page 133. 



In the septa of ammonites that portion of tlie septum which is nearest the umbilicus 

 is the simplest portion, and repeats more or less fully the condition of the whole septum 

 of the young and of ancestral moi'e primitive types; Placenticeras, Figs. 117-121. As 

 the septum is followed from the umbilicus outward to the ventral border a progressively 

 increasing complication takes place, which is parallel to the progressive complication seen 

 in passing from the young to the adult and concurrently from the geologically earlier to 

 the more recent types. An important feature in this case is the fact that the whole 

 septum is built at one time, representing a single period of growth. Its simpler and more 

 complicated portion, therefore, being built at one time, differ from the simple and more 

 complicated condition of the individual plates of Arbac^a and Strongylocentrotus (PI. 24, 

 figs. 110-116), in whicli ditt'erentiation takes place during the growth of the plate as 

 described. 



In the trilobite, TrUirthrns beckii, localized stages seem to be expressed in the distri- 

 bution of the ventral appendages, which posteriorly are )n-oad and essentially primitive, 

 Avhile anteriorly they are narrow and specialized (Beecher, '90) . 



In general, it may be said of localized reversionary stages of adult organisms, that 

 they do not repeat embryonic characters, but eai'lier or later nepionic or neanic characters, 

 as noted in numerous cases of plants and animals. Tlie chief interest of localized stages in 

 develo])ment is the fact, that they call for a comparative study of all parts of the 

 organism, young, adult, and old, and a coi'relation of these comparisons with the characters 

 in other living and fossil species. 



