REPORT ON THE ECHINOIDEA. 21 



analysis. These types thus seem on that account to have been introduced suddenly, 

 perhaps, from the great prominence assumed by any one of the Echinodermal structural 

 characters. Characters of very different degree of prominence in the older tj-pes, which, 

 slightly modified, might radically affect the structural features of any group. 



The modifications of the anal system, of the genital and ocular plates of the test, of 

 the poriferous zone of the actinal system, of the jaws, and so on, do not go on pari jyassu, 

 but, on the contrary, vary not only in every sub-order, but in every family and genus ; con- 

 sequently, modifications of the coronal plates which affect greatly the outline of the test, 

 and culminate in the Spatangoids, may be combined with other features of very little 

 systematic value in that group such as the position of the anal system. In the same way 

 in the Clypeastroids, the structure of the ambulacra, so widely different from that of the 

 Desmosticha, is found combined with jaws, and again in the Spatangoids a very simple 

 ambulacral system may be combined with an arrangement of the coronal plates, showing 

 the greatest degree of specialisation. We cannot hope, therefore, to trace the develop- 

 ment of any type through a series of forms, each slightly different from its predecessor ; 

 we must only expect to be able to follow the changes of a single feature, and study it 

 in its combinations with other features, combinations which from their very nature can 

 never form an unbroken series, as their terms are not synchronous ; combinations which 

 can never be links in any chain beyond the link formed by any one special character in 

 tracing its modifications alone. 



It is only in the Archseocidaridse that we find in the structure of the poriferous zone 

 structural features which have remained unaltered to the present day in all recent 

 Spatangoids, viz., the simple pairs of pores which we find in the Cidaridse, in the Clypeas- 

 troids, and in the Spatangoids ; in the Palseechinidse we find small tubercles characterising 

 a whole group of genera ; this structural feature, the absence of permanent primary 

 tubercles, still exists at the present day in the highest of our Petalosticha, and is found 

 uninterruptedly in genera living from the oldest time to the present day. 



The apical system which we find in the Palseechinidse still occurs at the present day, 

 but little modified in the Cidaridse, Arbaciadse, Diadematidse, and the tendency to throw 

 the anal system outside of this system is already hinted at in the excentric position of 

 the anal opening in the anal system ; and the compact abactinal system so characteristic 

 of the recent Spatangoids and Clypeastroids is already foreshadowed in the encroach- 

 ment of the madreporite on several of the genital plates. The specialisation of the plates 

 of the two areas, which takes its greatest development in the petaloid ambulacra of the 

 Petalosticha, can also be traced in a rudimentary form in the double ocular pore of the 

 ocular plate of the abactinal system, the structural features characteristic of the mode of 

 junction of the coronal plates of the Desmosticha, and of the Petalosticha, is also to be 

 traced in the construction of the coronal plates of the test of the Palasechinidse. The 

 characteristic subdivisions of the test of the Desmosticha into actinostome, coronal and 



