426 



NATURE 



[Sept. 2, iSSov 



We can trace in this genus the origin of the modifications of 

 the poriferous zone, leading us, on the one side, through gene/a 

 with merely undulating lines of pores to more or less distinct 

 confluent arcs of pores, formed round the primary ambulacral 

 tubercles, and, on the other, to the formation of open arcs of 

 three or more pairs of pores. The first type culminates at the 

 present day with the Arbaciadte, the otlier with the Diadematidtc, 

 Triplechinida;, and Ecliinometradx. This specialisation very 

 early takes place, for already in the lower Jura Stomechinus has 

 assumed the principal characteristics of the Triplechinidcc of to- 

 day. 



Although in Hemicidaris the number of the coronal plates has 

 increased as compared with the Cidarida:, and while we find tliat 

 in many genera, even of those of the present day, the number 

 of the coronal plates is still comparatively small, yet, as a general 

 rule, tlie more recent formations contain genera in which the 

 increase in number of the interambulacral plates is accompanied 

 by a corresponding decrease in the number of plates of the 

 interambulacral area so characteristic thus far of the CidariiUe 

 and HemicidaridK, a change also affecting the size of the primary 

 ambulacral tubercles. This increase in the number of the 

 coronal plates is likewise accompanied by the development of 

 irregular secondary and miliary tubercles, and the disappearance 

 in this group of tlie granular tuberculation, so important a 

 character in th- Cidarid.-e. Witlx the increase in the number of 

 the interambulacral coronal plates, the Pseudodiadematidx still 

 retain prominent primary tubercules, recalling the earlier Hemi- 

 cidarida; and Cidarida;, and, as in the Cidarid.-e proper, the test 

 is frequently ornamented by deep pits or by ridges formed by 

 the junction of adjoining tubercles. The genital ring becomes 

 narrower, and the tendency to the specialisation of one of its 

 plates, the madreporite, more and more marked. 



With the appearance of Stomechinus, the Echinidas proper 

 already assume in the Jura the open arcs of pores, the large 

 number of coronal interambulacral plates, the specialisation of 

 the secondary tubercles, and the large number of primary 

 tubercles in each plate. With the appearance of Sphcerechinus 

 in the early Tertiary come in all the elements for the greater 

 multiplication of the pairs of pores in the arcs of the poriferous 

 zones, while the gigantic primary spines of some of the genera 

 (Heterocentrotus), and the small number of primaiy tubercles 

 are stractural features which had completely disappeared in tlie 

 group preceding the Echinometradre, to which they appear most 

 closely allied. 



Going back again to the Hemicidaridce, it requires but slight 

 changes to pass from them to Acrosalenia and to the Salenia; 

 proper; the latter have continued to the present day, and have, 

 like the Cidarida;, retained almost unchanged the characters of 

 the genera which preceded them, combined however with a few 

 Cidaridian and Echinid features which date back to the Trias^ic 

 period. We can thus trace the modifications which have ta!;eu 

 place in the poriferous zone, the apical and actinal systems, the 

 coronal plates, the ambulacral and interambulacral tubercles, as 

 well as in the radioles, and in the most direct manner possible 

 indicate the origin of the peculiar combination of structural 

 features which we find at any geological horizon. On taking in 

 succession the modifications undergone by the different parts of 

 the test, we can trace each one singly, without the endless com- 

 plication of combinations which any attempt to trace the whole 

 of any special generic combination would imply. 



Leaving out of the question for the moment the Palcechinida?, 

 we find no difficulty in tracing the history of the characters of 

 the genera of the regular Echini iihich have existed from the 

 time of the Trias and are now living, provided we take up each 

 character independently. Nothing can be more direct than the 

 gradual modification of the simple, barely undulating poriferous 

 zone, made up of numerous ambulacral plates covered by 

 granules, such as we find it among the Cidarida: of the 

 Trias, first into the slightly undulating poriferous zone of the 

 Hemicidarida;, next into the indistinct arcs of pores of the 

 Pseudodiadematida;, then into the arcs with a limited number of 

 pores of the Triplechinidce, and finally to the polyporous arcs of 

 the Echinometradx. What can be more direct than the gradual 

 modification to be traced in the development of the primary 

 ambulacral tubercles, such as are characteristic of the Echinida; 

 of the present day, from their first appearance at the oral 

 extremity of the ambulacral system of the Hemicidarida?, and 

 the increase in the number of primary interambulacral tubercles, 

 accompanied by the growth of secondaries and miliaries, which 

 we can trace in Hemicidaris, Acrosalenia, and Stomechinus— the 



increase m number of the primary and secondary tubercles being- 

 accompanied by a reduction in the size of the radioles and a' 

 greater uniformity in their size and shape ? 



But while these modifications take place the original structural 

 feature may be retained in an allied group. Thus the Cidarida; 

 retain unchanged from the earliest time to the present day the 

 few primary tubercles, the secondary granules, the simple 

 poriferous zone, the imbricating actinal system, and the few- 

 coronal plates, with the large apical system and many-shaped 

 radioles; while in the Salenidre the primaiy interambulacral 

 tubercles, the secondary gi-anules, the radioles, the genital ring, 

 are recognised features of the Cidaridce, associated, however, 

 \iith an Echinid actinal and anal system, Plemicidarid primary 

 ambulacral tubercles, and an Echinid poriferous zone. In the 

 same way in the Diadematidx, the large primary interambulacral 

 tubercles are Cidaridian features, while the structure of the 

 ambulacral tubercles is Hemicidaridian. The existence of two 

 kinds of spines is another Cidaridian feature, while the apical 

 and actinal systems have become modified in the same direction 

 as that of the Echinida?. The more recent the genus 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 maybe derived 

 from totally different sources. This is particularly the ease 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 supjiosition 

 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 m hich have preceded this 

 comliination 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 d.ay in some generic type of the present epoch, 

 fully as well characterised as it was in the beginning. In fact, 

 the very existence today of these early structural features seems 

 to be as positive a proof of the unbroken systematic affinity 

 between the Echini of our seas and those of the Trias as the 

 uninterrupted existence of the genus Pygaster orCidaris from the 

 Trias down to the present epoch, or of the connection of many 

 of the genera of the Chalk with those of our epoch (Salenia, 

 Cyphosoma, Psammechinus, &c.). 



Passing to the Clypeastridre, we find there, as among the 

 Desmoslicha, that the earliest type, Pygaster, has existed from 

 the Trias to the present time ; and that, while we can readily re- 

 construct, on embryological grounds, the modifications theearlies;t 

 Desmosticha-like Echini should undergo in order to assume the 

 structural features of Pygaster, yet the early periods in which 

 the precursors of the Echinoconida; and Clypeastrida? are found 

 have thus far not produced the genera in which these modifica- 

 tions actually take place. But, starting from Pygaster, we 

 naturally pass to Ilolectypns, to Discoidea, to Conoclypus, on 

 the one side, while on the other, from Holectypus to Echino- 

 cyamus, Sismondia, Fibularia, and Mortonia, we have the natural 

 sequence of the characters of the existing Echinanthida?, 

 Laganida?, and Scutellida?, the greater number of which are 

 characteristic of the present epoch. If we were to take in turn 

 the changes undergone in the arrangement of the plates of the test, 

 as we jiass from Pygaster to Holectypus, to Echinocyamu*, and 

 the Echinanthidx, we should have in the genera which follow 

 each other in the pala?ontological record an unbroken series 

 showing exactly what these modifications have been. In the 

 same way the modifications of the abactinal and anal systems, 

 and those of the poriferous zone, can equally well be followed 

 to Echinocyamus, and thence to the Clypeastrida? ; while a 

 similar sequence in the modifications of these structural features 

 can be followed from Mortonia to the Scutellida; of the present 

 period. 



Passing finally to the Petalosticha, we find no difficulty in 

 tracing theoretically the modifications which our early Echino- 

 conida? of the Lias should primatily undergo previous to the 

 appearance of Galeropygus. The similarity of the early Cassi- 

 duloid and Echinoneoid types points to the same systematic 



