INTRODUCTION. 17 



fig. 3), which progressively adds characters, but as far as known does not lose any of these 

 additions. It may be said that in general most Palaeozoic Echini arc progressive types. 



Regressive types are those whicih, after attaining a degree of specialized characters, in 

 later development, and before old age, lose some of these characters, so Ihat what we call the 

 adult, as gathered especially from the characters of the mid-zone, is simpler than its own young. 

 An excellent case of this is Lepidesthes wortheni (Plate 67, fig. S) which in the young has four 

 columns of interambulacral plates, but the fourth column drops out earlj^, and in later life it 

 has three columns only. The sixth column represented by only a few plates in an area seems 

 to mark Lnvenechimis missouriensis (Plate 41, fig. 1) as a regressive type, at least in this char- 

 acter. To go outside of Palaeozoic Echini, Homicidaris has compound ambulacral plates 

 in the lower or youthful half of the test, and above this point has only simple plates, showing 

 a complete reversion to Cidaris, a more primitive type of Echini. Such an extreme case ma}' 

 be compared to Lituites in cephalopods (Zittel, Handbuch der Palaeontologie, vol. 2, text- 

 fig. 519), which, after an early coiled stage, takes on a straight stage directly comparable to 

 the early straight forms of nautiloid cephalopods. Cases could be multiplied and some will be 

 mentioned later, but here it is the object simply to point out examples that illustrate the 

 principles made use of. 



Acceleration of development, one of Professor Hyatt's most important principles, is abun- 

 dantly shown in Palaeozoic Echini. U.sually columns of interambulacral plates, after the 

 first four columns, are added at considerable intervals, as in Melonechinus muUiporus (Plate 

 57, fig. 1), l)ut M. giganteus (Plate 59, fig. 14), which is a higher species in the series, adds the 

 fifth, sixth, seventh, and eighth columns earlier than does M. muUiporus, as shown in a detailed 

 study in my earlier paper (189(5, p. 179). Hyattechinus beecheri (Plate 26), a most specialized 

 type, has a very accelerated development, and new columns of interambulacral plates are added 

 so rapidly that the fourth to the tenth are added in succeeding rows, or even two columns may 

 be added in a single row. The same character of acceleration is shown well in Hyattechinus rari- 

 spinus (Plate 23, fig. 1) and H. pentagonus (Plate 25, fig. 1). Acceleration is shown well in the 

 •ambulacrum in Melonechinus (text-fig. 237, p. 231), in which at the ventral border we find 

 four plates in each ambulacrum, while the lower genera of its family have only two plates. 



Parallelism is an important feature which was much studied by Professor Hyatt, and is 

 of great value in studying Echini. Parallelism is the taking on of a similar character by inde- 

 pendent lines, and is sometimes difficult to distinguish from real genetic connection. A case 

 of parallelism is .seen in the imbrication of plates of Echini. It has been thought that imbri- 

 cation was a sufficiently important character to group together those forms that possessed it, 

 but it is assumed independentlj' in several distinct groups in the Palaeozoic, as well as in the 

 post-Palaeozoic Echinothuriidae. In the family of the Palaeechinidae (Melonechinus, etc.) 

 ambulacral plates on the adradial suture are beveled over the adambulacrals (Plate 45, fig. 5). 



