400 H. L. Jfaivkins — Ecliinoidea Holectypoida. 



tuberculation becomes amplified in the hour-glass pattern 

 characteristic of Conulus. (In E. sternopetala Ag. & Clark, 

 " Conulus-tuhexculation " seems persistent even in the adult.) It 

 is in the ambulacra, however, that the young Echinolampas exhibits 

 the most remarkable qualities. Agassiz says of these (I.e., p. 742), 

 " each plate . . . carries a single primary tubercle . . . The pores 

 are arranged . . . three or four for each plate." His enlarged drawings 

 (here PI. VII, Fig. 5) show that " each plate " is really a triad-group. 

 No indication appears as to the relations of the components of the 

 groups, but that they are not all primaries seems probable from the 

 nature of the perradial suture. In view of Agassiz' previously 

 quoted remark, it is reasonable to assume that the triad-groups 

 are Pyrinid in character, like those of Echinoneus. Such triadic 

 structure is persistent in Amhly2)ygus (otherwise very nearly akin 

 to Echinolamqjas), but it is not retained in adults of Ecliinolam/pas 

 itself. Its presence in early ontogeny must surely be vestigial. 

 In consequence the descent of the Echinolampas-stock from an 

 Echinoneid (or late Holectypoid) ancestry is a conception that is 

 now so far established that it is for those who disagree to disprove it. 

 Stratigraphy, morphology, and now ontogeny, all point to its truth. 

 The ambulacral history of the Nucleolitoida and Cassiduloida 

 (assuming the above conclusions to be accurate) shows a strange ' 

 course of morphogeny. The phyletic relations of the groups, as 

 they appear to me, can be indicated thus : — 



Lower Jura | Upper Jura | Cretaceous ] Tertiary 



~ Clypeus Pygurus \ |- Nucleolitoida. 



Galeropygus-^Nucleolites—Trematopygus—A'paiopygus ) 



Plesiechinus 



"'■■■■: Pygaster ? " Pyrina" Echinoneus Holectypoida. 



""■ Ecliinolam'pas Cassiduloida. 



The Nucleolitoida early acquired elaborate phyllodes, retaining 

 simplicity in the rest of their ambulacra. During the Mesozoic era 

 their phyllodes underwent gradual decentralization, until triad- 

 grouping afiected a large part of the areas. This tendency culminates 

 in AjMtopygus. On the other hand, the Holectypoida failed to 

 develoj) phyllodes, but progressively spread triad-grouping over 

 their ambulacra (e.g. " Pyrina "). In the Tertiary era, simpli- 

 fication of plating, coupled with production of well-defined 

 phyllodes (perhaps foreshadowed in Conulus), characterized the 

 Cassiduloida. The two independent trends separated rapidly in 

 the Lower Jurassic, and then slowly converged until, in the 

 Cretaceous, they were almost coincident. Modern Cassiduloida 

 have reached a stage analogous with that of the precocious Clypeidee 

 of the Nucleolitoida. But Apatopygus and Echinoneus remain as 

 superannuated survivors, the last of their respective orders, 

 wearing to-day the ambulacral fashions of the Cretaceous period. 



