REPORT ON THE ECIIINOIDEA. 23 



plastron, of an anal snout, of a beak or rostrum, and the formation of an actinal lip ; 

 connected with these beaks and plastrons are the accumulation along certain lines 

 of bands of miliaries, the fascioles, which can be traced far back in the formation of the 

 so-called miliaries in the first j)lace, and their accumulation at certain points, and finally, 

 their enclosing certain definite areas. If we trace the existence of the slight groove of 

 the anterior part of the test, we go back to the Chalk, and the time when the odd 

 posterior ambulacra began to be developed at a difi'erent rate from tlie others, and to 

 retain its primitive character. The petaloid structure of the lateral ambulacra dates back 

 to the Jurassic period, when the ambulacral areas above the ambitus differed in their propor- 

 tions from those of the actinal surface of the test. The petaloid structure of the ambulacra 

 adjoining the actinostome began with the oldest Cassidulidse, and the simple ambulacral 

 pores which connect the actinal and petaloid ambulacral plates we find in a part of the 

 ambulacral zone in the earliest Cretaceous Spatangoids. But the other characters with 

 which the various structural features still found in Spatangus are connected in the 

 older genera where they occur are of a very diSerent degree of intensity, and have many 

 of them developed in directions which no longer occur, and have formed types which 

 have become extinct, though the special structure which has been modified still exists. 



The peculiar internal appendage which represents the auricles in Spatangoids, and the 

 whole dental system, is reduced to a simple spur, and is the only trace of the complicated 

 dental aj^paratus which we find in the oldest known Echinids, and which in another 

 direction has remained but little modified up to the present day. Taking in a similar 

 way one of the most characteristic of the older genera, Ananchytes, we can also trace 

 backwards to their first appearance, as we have done for Spatangus, the genera in which 

 the characteristics of the genus Ananchytes are first developed ; but we can likewise trace 

 in Ananchytes its affinities to the recent Spatangoid genera, and find in the structure of 

 the apical system of the amljulacra, of the anal system, of the actinal plastron, and of 

 the actinostome, indications of lines of development which date back to the genus 

 Ancmchytes, and which are still to be traced at the present day even though Ananchytes is 

 at the present time extinct. Another such characteristic genus is Pygaster, in which the 

 whole line of the Clypeastroids is to a certain extent foreshadowed, although if we 

 compare the Clypeastroids to the earlier Echinids, we shaU find a far greater number of 

 identical points of structure than when comparing such a recent genus as Spiatangus. 



Adopting the other method, and tracing the development of a single structural feature 

 at a time, such as the growth of the poriferous zone, from the simple paired zone to the com- 

 plicated ambulacral zone of a Spatangoid, we shall find that the most primitive ambulacra! 

 zone known still exists side by side with the existence at the present day of the resultants, 

 if we may so say, of all the combinations which have taken place. In the same way the 

 earliest modifications of the coronal plates of the Palseechinidse are found to-day to exist 

 in several of the recent Desmosticha, along with a test made up of such a sj)ecialised set of 



