INTRODUCTION. 17 



fig. 3), which progressively adds characters, but as fur ;is known doc- not ]<< any of thee 

 additions. It may be said that in general must Palaeo/oie Kchini arc pn.grc--i\r i , 



Regressive types are those which, after attaining a degree of speciali/ed characi, , 

 later development, and before old age, lose some of these characters. BO that what we call the 

 adult, as gathered especially from the characters of the mid-xone, i* -i m ,,!,.,- t |, ;m ;,. ,, wh yitin , v 

 An excellent case of this is Lepidesthes wortheni (Plate (17, fig. 8) which in the young ha.- lour 

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

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

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

 acter. To go outside of Palaeozoic Echini, Hemieidaris has compound ambulacral pi: 

 in the lower or youthful half of the test, and above this point has only simple plate-. -Inming 

 a complete reversion to Cidaris, a more primitive type of Kchini. Such an extreme ca-e may 

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

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

 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 principle-, i- abun- 

 dantly shown in Palaeozoic Echini. Usually columns of interambulacral plates, after t In- 

 first four columns, are added at considerable intervals, as in Melonechimu ninlti/>i>rux (Plate 

 57, fig. 1), but M. giganteus (Plate 59, fig. 14), which is a higher species in the serie-. adds tin- 

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

 study in my earlier paper (1896, p. 179). Hyattechinus beecheri (Plate 26), a mo-t -peciali/ed 

 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 column- may 

 be added in a single row. The same character of acceleration is shown well in Ili/nlli-cliinitx rnri- 

 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 plat. 



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 po-e-ed it. 

 but it is assumed independently in several distinct groups in the Palaeozoic, a- well as in the 

 post-Palaeozoic Echinothuriidae. In the family of the Palaeechinidae (Melonechimis. . 

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



