SCIENCE. 



143 



young Echini has been traced in a sufficient number of 

 tamilies to enable me to draw the parallelism between these 

 various stages of growth and the paleontological stages in 

 a very different manner from what is possible in other 

 groups of the animal kingdom, where we are overwhelmed 

 with the number of species, as in the Insects or Mollusks, 

 or where the paleontological or the embryological terms of 

 comparison are wanting or very imperfect. 



Beginning with the paleontological history of the regular 

 Sea-urchins of the time of the Trias, when they constituted 

 an unimportant group as compared with the Crenoids, we 

 find the Echini of that time limited to representatives of 

 two families. One of these, the genus Cidaris, has con- 

 tinued to exist, with slight modifications, up to the present 

 time, and not less than one-tenth of all the known species 

 of fossil Echini belong to this important genus, which in 

 our tropical seas is still a prominent one. It is interesting 

 here to note that in the Cidaridae the modifications of the 

 test are not striking, and the fossil genera appearing in the 

 successive formations are distinguished by characters which 

 often leave us in doubt as to the genus to which many 

 species should be referred. In the genus Rhabdocidaris, 

 which appears in the lower Jura, and which is mainly char- 

 acterized by the extraordinary development of the radioles, 

 we find the extreme of the variations of the spines in this 

 family. From that time to the present day, the most strik- 

 ing differences have existed in the shape of the spines, not 

 only of closely allied genera, but even in specimens of the 

 same species ; differences which in some of the species of 

 to day are as great as in older geological periods. The 

 oldest Cidaridae are remarkable for their narrow poriferous 

 zones. It is only in the Jura that they widen somewhat ; 

 subsequently the pores become conjugated, and only later, 

 during the Cretaceous period, do we find the first traces of 

 any ornamentation of the test (Temnocidaris) so marked at 

 the present day in the genus Goniocidaris. As far, then, 

 as the Cidaridae are concerned, the modifications which 

 take place from their earliest appearance are restricted to 

 slight changes in the poriferous zone and in the ornamen- 

 tation of the test, accompanied with great variability in the 

 shape of the primary radioles. We must except from this 

 statement the genera Diplocidaris and Tetracidaris, to 

 which I shall refer again. The representatives of the other 

 Triassic family become extinct in the lower tertiaries. The 

 oldest genus, Hemicidaris, undoubtedly represents the 

 earliest deviations from the true Cidaris type ; modifica- 

 tions which affect not only the poriferous zone, but the test, 

 the actinal and the abactinal systems, while from the extent 

 of these minor changes we can trace out the gradual devel- 

 opment of some of the characteristics in families of the reg- 

 ular Echini now living. The genus Hemicidaris may be 

 considered as a Cidaris in which the poriferous zone is 

 narrow and undulating, in which the granules of the ambu- 

 lacral system have become minute tubercles in the upper 

 portion of the zone and small primary tubercles in its actinal 

 region, in which many of the interambulacral granules be- 

 come small secondaries, in which the plates of the actinal 

 system have become reduced in number, and the apical 

 system has become a narrow ring, and finally in which the 

 primary radioles no longer assume the fantastic shapes so 

 common among the Cidaridae. 



We can trace in this genus the origin of the modifications 

 of the poriferous zone, leading us, on the one side, through 

 genera 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 Arba- 

 ciadae, the other with the Diadematidae, Triplechinidae, and 

 Echinometradae. This specialization very early takes place, 

 for already in the lower Jura Stomechinus has assumed the 

 principal characteristics of the Triplechinidae of to-day. 



Although in Hemicidaris the number of the coronal 

 plates has increased as compared with the Cidaridse, and 

 while we find that in many genera, even of those of the 

 present day, the number of the coronal plates is still com- 

 paratively small, yet, as a general rule, the more recent 

 formations contain genera in which the increase in number 

 of the interambulacral plates is accompanied by a corre- 

 sponding decrease in the number of pla'.js of the interam- 

 bulacral area so characteristic thus far of the Cidaridae and 



Hemicidaridae, a change also affecting the size of the pri- 

 mary ambulacral tubercles. This increase in the number 

 of the coronal plates is likewise accompanied by the devel- 

 opment of irregular secondary and miliary tubercles, and 

 the disappearance in this group of the granular tubercula- 

 tion, so important a character in the Cidaridae. With the 

 increase in the number of the interambulacral coronal 

 plates, the Pseudodiadematidae still retain prominent pri- 

 mary tubercles, recalling the earlier Hemicidaridae and Cida- 

 ridae, and, as in the Cidaridae proper, the test is frequently 

 ornamented by deep pits or by ridges formed by the junc- 

 tion of adjoining tubercles. The genital ring becomes 

 narrower, and the tendency to the specialization of one of 

 its plates, the madreporite, more and more marked. 



With the appearance of Stomechinus, the Echinidae 

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

 the large number of coronal interambulacral plates, the 

 specialization of the secondary tubercles, and the large 

 number of primary tubercles in each plate. With the ap- 

 pearance of Sphaerechinus in the early Tertiary come in all 

 the elements lor 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 primary tubercles are structural 

 features which had completely disappeared in the group 

 preceding the Echinometradae, to which they appear most 

 closely allied. 



Going back again to the Hemicidaridae, it requires but 

 slight changes to pass from them to Acrosalenia and to the 

 Saleniae proper ; the latter have continued to the present 

 day, and have, like the Cidaridae, retained almost un- 

 changed the characters of the genera which preceded them, 

 combined, however, with a few Cidaridian and Echinid fea- 

 tures which date back to the Triassic period. We can thus 

 trace the modifications which have taken place in the pori- 

 ferous 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 pos- 

 sible 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 complication of combinations which 

 any attempt to trace the whole of any special generic combi- 

 nation would imply. 



Leaving out of the question for the moment the Palaechi- 

 nidae, we find no difficulty in tracing the history of the char- 

 acters of the genera of the regular Echini which have 

 existed from the time of the Trias and are now living, pro- 

 vided 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 Cidaridae of Trias, first into the 

 slightly undulating poriferous zone of the Hemicidaridae, 

 next into the indistinct arcs of pores of the Pseudodiade- 

 matidae, then into the arcs with a limited number of pores 

 of the Triplechinidae, and finally to the polyporous arcs of 

 the Echinometradae. 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 Echinidae of the present day, from their first appearance 

 at the oral extremity of the ambulacral system of the He- 

 micidaridae, and the increase in the number of primary in- 

 terambulacral tubercles, accompanied by the growth of 

 secondaries and miliaries, which we can trace in Hemicida- 

 ridis, Acrosalenia, and Stomechinus, — the increase irr 

 number of primary and secondary tubercles being accom- 

 panied 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 Cidaridae retain unchanged from the earliest time to the 

 present day the few primary tubercles, the secondary gran- 

 ules, 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 Salenidae 

 the primary internambulacral tubercles, the secondary 

 granules, the radioles, the genital ring, are recognized fea- 

 tures of the Cidaridae, associated, however, with an Echinid 

 actinal and anal system, Hemicidarid primary ambulacral 



