24 THE VOYAGE OF H.M.S. CHALLENGER. 



coronal plates as we find in Moira. The actinostome of the Palseechinidse still exists side 

 by side with the anomalous actinal system of Palceostoma and the Pourtalesise. and the 

 apical system of the Cidaridse is contemporaneous with the extraordinary combination of 

 the apical and anal system of the Pourtalesite, and spines very similar in structure to 

 those so characteristic of the Palgeechinidse are found in the same period with the spines 

 of the Cidaridse, ofthe Diadematidse, of the Clypeastroids, and of the Petalosticha. 



What has once been gained is never totally lost, it always reappears, not in the 

 previous form but in a slightly modified one, sufficiently preserved to show its systematic 

 connections, and hence the hopelessness of the task to do more than hint at the infinite 

 number of relations which the types of the present day hold to those which have preceded 

 them — relations which witli each succeeding formation become more and more difficult 

 to trace in proportion as our knowledge of the older formations is more accurate. 



The existence of teeth in most groups of the Echinoidea, no matter how distantly 

 related, is one of the most striking examples of the persistence of a structural feature 

 once introduced, and of its development or modification entirely independent of other 

 accompanying characters. The rate and direction of development of the teeth, of the 

 modifications of the ambulacral system, of the coronal plates, of the anal and actinal 

 systems, do not go on imri passu when once a slight modification has become introduced, 

 and thus it is that we have in some of the earlier groups, such as the CoUyritidas and 

 Clypeastridse, which exist side by side, a widely difi"erent degree of complication of 

 structure in the arrangement of the coronal plates, in the structure of the actinostome, 

 in that of the apical system, of the ambulacral system, which have all developed in 

 difiereut directions from their first origin, so as to produce in the one case the 

 CoUyritidae, and in the other the Clypeastridae. Thus it is that among the Clypeas- 

 troids we find genera with very powerful jaws side by side with genera in which the jaws 

 attain but a slight development, whde other characteristic features of the group, such as 

 tlie arrangement of the coronal plates, the degree of specialisation of the petaloid 

 ambulacra, and the structure of the apical system, may be nearly equally developed. 



Thus it is that among some of the Echinolampadse we find a prominent auricular ring, 

 while it is wanting in closely alhed genera. In the same way, in some Spatangina, the 

 large spur developed close to the actinostome in some genera is not found in theii" closest 

 allies. It is to this same variation in the degree of development of the ambulacral system, 

 combined with a difi"erently developed anal system, coronal plates, fascioles, and actino- 

 stome, that we owe the great diversity we find in the recent genera of Spatangoids, and 

 it is to their predecessors in time in the Tertiaries, the Chalk, the Jura or often far earlier, 

 that we must look for the appearance of the structural features which, with the special com- 

 binations of structural features which may exist at any one period, give us the facies of the 

 time. We must remember, while making our comparisons, that these structural featui-es, 

 when once they have originated, may either continue as a persistent type of structure 



