SEA-URCHINS. 441 



of tlie Ecliiiioidea. To a casual observer, unacquainted with the 

 anatomy and embryology of the forms, such genera as Cldarifi -dwd 

 Edunocardimn would seem to represent two absolutely difierent 

 '■ kinds " of animals. Moreover, search as he might among the 

 Echinoids of the present dny, he could find no types that would 

 ettectively l)ridge the gulf that separates the Regular from the 

 Irregular Sea- Urchins. All the Regularia have thick tests, built 

 on a radially symmetrical plan ; possess a strong jaw-apparatus ; 

 and are armed with stout, often very long, radioles. On the ether 

 han<l, most of the Irregularia have thin tests, bilateially sym- 

 metrical ; have no jaws ; and are covered with small, almost 

 hair-like radioles. Even those forms, the " Cake-Urchins," which 

 show a radial symmetry and possess jaws, have other characters 

 Avhich render them very difiei-ent in appearance from the 

 Regular Echinoids. 



And again, from an anatomical study of living forms alone, 

 although a similarity of structure suliicient to warrant their 

 inclusion in the same order of the Echinoderma might be found, 

 no certain clues as to the relation of the two types to one another 

 could be discovered. Even ontogenetic evidence is lacking to a 

 considerable degree, for the peculiar larval life led by Echinoids 

 has tended to neuti-alise the effect of recapitulation. 



At this point, where Zoology in its narrower sense fails, 

 Paljeontology can supply the connecting links between such 

 different foi-ms as a Cidarid and a Spatangid ; and of these links, 

 the majority are to be found in the group which forms the 

 subject of this research. The order Holectypoida arose soon after 

 the commencement of the Jurassic period, and became entirely 

 extinct before the close of the Mesozoic era. During its existence 

 it gave rise to forms wdiich, by stages so gradual as to be hardly 

 distinguishable, laid the foundations of all the great groups of 

 Irregular Echinoids that are living to-day. It is the purpose of 

 this paper to indicate in outline the processes through which the 

 specialization of Clypeastroids, 8patangoids, and '' Cassidulids " 

 was achieved. 



When P. M. Duncan (44)* wrote his classic "Revision of 

 the Genera and great Groups of the Echinoidta,"' there existed 

 " much diversity of opinion regarding the nature of the peri- 

 gnathic girdles and jaws of some genera" {t.c, p. 135) of the 

 Holectypoida, the most primitive group of the Irregular 

 Echinoids, As Duncan's system of classification was based to a 

 very large extent on the varying characters of these structures, it 

 naturally resulted that in the case of the Holectypoida and their 

 allies the grouping of genera into families, or even into lai'ger 

 groups, was somewhat tentative. It is only necessary to remark 

 that jaws are now known to have existed in two {genera, in which 

 their presence was denied by Duncan, to show that a revision of 

 the classification of the gioup is required. 



* VVlierea nuniber in brackrts follows the name of an author, the full title of the 

 paper refcneil to will he found under that number in the chronological list of 

 literature at the end of this paper. 



