and Embryological Development. 355 



of the Echinidre. The more recent the germs, the greater is 

 the difficulty of tracing in a direct manner the origin of any 

 one structural feature, owing to the difficulty of disassociating 

 structural elements characteristic of genera which may be 

 derived from totally different sources. This is particularly 

 the case with genera having a great geological age. Many 

 of them, especially among the Spatangoids, show affinities 

 with genera following them in time, to be explained at present 

 only on the supposition that, when a structural feature has 

 once made its appearance, it may reappear subsequently, 

 apparently as a new creation, while, in reality, it is only its 

 peculiar combination with structural features with which it 

 had not before been associated (a new genus) which conceals 

 in that instance the fact of its previous existence. A careful 

 analysis, not only of the genera of the order, but sometimes 

 of other orders which have preceded this combination in time, 

 may often reveal the elements from which have been produced 

 apparently unintelligible modifications. 



There is, however, not one of the simple structural features 

 in the few types of the Triassic and Liassic Echini from which 

 we can so easily trace the origin of the structural features of 

 all the subsequent Echinid genera, which is not also itself 

 continued to the present day in some generic type of the 

 present epoch, fully as well characterized as it was at the 

 beginning. In fact, the very existence to-day of these early 

 structural features seems to be as positive a proof of the un- 

 broken systematic affinity between the Echini of our seas and 

 those of the Trias, as the uninterrupted existence of the genus 

 Pygaster or Cidaris from the Trias down to the present 

 epoch, or as the connexion of many of the genera of the 

 Chalk with those of our epoch (Salenia, Cyphoso?7ia } Psam- 

 mechmuSj &c). 



Passing to the Clypeastrida?, we find there, as among the 

 Desmosticha, that the earliest type, Pygaster, has existed 

 from the Trias to the present time, and that while we can 

 readily reconstruct, on embryological grounds, the modifica- 

 tions the earliest Desmosticha-like Echini should undergo 

 in order to assume the structural features of Pygaster, yet the 

 early periods in which the precursors of the Echinoconidre 

 and Clypeastridae are found have thus far not produced the 

 genera in which these modifications actually take place. But, 

 starting from Pygaster, we naturally pass to Holectypus, to 

 Discoidea, to Conoclypus, on the one side ; while, on the other, 

 from Holectypus to Echinocyamus, Sismondia, Fibularia, and 

 Mortonia we have the natural sequence of the characters of 

 the existing Echinanthidse, Laganidge, and Scutellidae, the 



