146 



SCIENCE. 



veloped last among the Echinidae of the present day. Very 

 much in the same manner as a young Echinus develops, 

 they lose, little by little, first their Cidaridian affinities, 

 which become more and more indefinite, next their Didem- 

 atidian affinities, if I may so call the young stages to which 

 they are most closely allied, and, finally, with the increase 

 in the number of the coronal plates, the great numerical 

 development of the primary tubercles and spines, and that 

 of the secondaries and miliaries which we can trace in the 

 fossil Echini of the Tertiaries, we pass insensibly into the 

 generic types characteristic of the present day. 



Although we know nothing of the embryology of the 

 Salenidae, yet, like the Cidaridae, they have in a great meas- 

 ure remained a persistent type, the modifications of the 

 group being all in the same direction as those noticed in 

 the other Desmosticha ; a greater number of coronal plates ; 

 the development of secondaries and miliaries combined 

 with a specialization of the actinal system not found in the 

 Cidaridae. 



An examination of the succession of the Echinoconidae 

 shows but little modification from the earliest types ; the 

 changes, however, are similiar to those undergone by the 

 Clypeastroids and Petalosticha, though they do not extend 

 to modifications of the poriferous zone, but are mainly 

 changes in the actinostome and in the tuberculation. In 

 fact, the group of Echinoconidae seems to hold somewhat 

 the same relation to the Clypeastroids which the Salenidae 

 hold to the Cidaridae, and the earliest genus of the group 

 (Pygaster) has remained, like Cidaris, a persistent type to 

 the present day. 



The earliest Clypeastroids are all forms which resemble 

 the Fibularina and the genera following Echinocyamus and 

 Fibularia ; they are mainly characterized by the same 

 changes which an Echinarachniusor a Mellita, for instance, 

 undergoes as it passes from its Echinocyamus stage to the 

 Laganum or Encope stage. The comparison is somewhat 

 more complicated when we come to the Spatangoids. The 

 comparison of the succession of genera in the different 

 families, as traced in the Desmosticha and Clypeastroids, 

 is made difficult from the persistency of the types preceding 

 the Echinoneidae and the Ananchytidae, which have re- 

 mained without important modifications from the time of 

 the lower Cretaceous ; previous to that time the modifica- 

 tions of the Cassidulidae are found to agree with the 

 changes which have been observed in the growth of Echin- 

 olampas. The early genera, like Pygurus, have many of 

 the characteristics of the test of the young Echinolampas. 

 The development of prominent bourrelets and of the flos- 

 celle and petals goes on side by side with that of genera in 

 which the modification of the actinostome, of the test, and 

 of the petals is far less rapid, one group retaining the 

 Echinoneus features, the other culminating in the Echino- 

 lampas of the present day, and having likewise a persistent 

 type, Echinobrissus, which has remained with its main 

 structural features unchanged from the Jura to the present 

 day. That is, we find genera of the Cassidulidae which 

 recall the early Echinoneus stage of Echinolampas, next 

 the Caratomus stage, after which the floscelle, bourrelets, 

 and petals of the group become more prominent features 

 of the succeeding genera. Accompanying the persistent 

 type Echinobrissus, genera appear in which either the 

 bourrelets or petals have undergone modifications more 

 extensive than those of the same parts in the genera of the 

 Echinoneus or Caratomus type. 



The earliest Spatangoids belong to the Dysasteridae, ap- 

 parently an aberrant group, but which, from the history of 

 the young Hemiaster.we now knowto be a strictly embryonic 

 type, which, while it thus has affinities with the true Spat- 

 angoids, still retains features of the Cassidulidae in the 

 mode of development of the actinostome and of the petals, 

 as well as of the anal system. The genera following this 

 group, Holaster and Toxaster, can be well compared, the 

 one to the young stages of Spatangus proper before the ap- 

 pearance of the petals, when the ambulacra are flush with the 

 test, and when its test is more or less ovoid, the other to a 

 somewhat more advanced stage, when the petals have 

 made their appearance as semi-petals. In both cases the 

 actinostome has the simple structure characteristic of all 

 the young Spatangoids. The changes we notice in the gen- 

 era which follow them lead in the one case through very 

 slight modifications of the abactinal system, of the anterior 



and posterior extremities of the test, to the Ananchytid- 

 like Spatangoids of the present day, the Pourtalesiae, the 

 genus Holaster itself persisting till well into the middle of 

 the Tertiary period ; while on the other side we readily re- 

 cognize in the Spatanginae which follow Toxaster (a per- 

 sistent type which has continued as Palaeostoma to the 

 present day) the genera which correspond to the young 

 stages of such Spatangoids as Spatangus and Brissopsis 

 of the present day, genera which, on the one hand, lead from 

 Hemiaster (itself still represented in the present epoch), 

 through stages such as Cyclaster, Peripneustes, Brissus, 

 and Schizaster, and, on the other, through Micraster and 

 the like, to the Spatangoids, in which the development of 

 the anal plastron and fasciole performs an important part, 

 while in the former group the development of the peripe- 

 talus fasciole and of the lateral fasciole can be followed. 

 None of the genera of Petalosticha belonging to the other 

 groups devolop any fasciole in the sense of cirumscribing 

 a limited area of the test. 



The comparison of the genera of Echini which have ap- 

 peared since the Lias with the young stages of growth of 

 the principal families of Echini, shows a most striking co- 

 incidence amounting almost to identity between the suc- 

 cessive fossil genera and the various stages of growth. 

 This identity can, however, not be traced exactly in the 

 way in which it has usually been understood, while there 

 undoubtedly exists in the genera which have appeared one 

 after the other a gradual increase in certain families in the 

 number of forms, and a constant approach in each succeed- 

 ing formation, in the structure of the genera, to those of the 

 present day. It is only in the accordance between some 

 special points of structure of these genera and the young 

 stages of the Echini of the present day that we can trace an 

 agreement which, as we go further back in time, becomes 

 more and more limited. We are either compelled to seek 

 for the origin of many structural features in types of which 

 we have no record, or else we must attempt to find them 

 existing potentially in groups where we had as yet not suc- 

 ceeded in tracing them. The parallelism we have traced 

 does not extend to the structure as a whole. What we find 

 is the appearance among the fossil genera of certain struc- 

 tural features giving to the particular stages we are com- 

 paring their characteristic aspect. Thus, in the succession 

 of the fossil genera, when a structural feature has once 

 made its appearance, it may either remain as a persistent 

 structure, or it may become gradually modified in the suc- 

 ceeding genera of the same family, or it may appear 

 in another family, associated with other more marked 

 structural features which completely overshadow it. 

 Take, for instance, among the Demosticha, the modifi- 

 cations of the poriferous zone of the actinal and ab- 

 actinal systems of the coronal plates, of the ambulachral 

 and interambulachral systems, the changes in the relative 

 proportion of the primary-tubercles, and the development 

 of the secondaries. These are all structural features which 

 are modified independently one of the other ; we may find 

 simultaneous development of these features in parallel 

 lines, but a very different degree of development of any 

 special feature in separate families. 



This is as plainly shown in the embryological as in the 

 paleontological development. In the Cidaridae there is the 

 minimum of specialization in these structural features. In 

 the Diademopsidae there is a greater range in the diversity 

 of the structure of the poriferous zone and of the coronal 

 plates, as well as of the actinal system. There is a still 

 greater range among the Echinidae, while among the Sal- 

 enidae the modifications, as compared to those of the 

 Echinidae and Diademopsidae, are somewhat limited again, 

 being restricted as far as relates to the poriferous zone and 

 coronal plates, but specialized as far as the actinal system 

 is concerned, and specially important with reference to the 

 structure of the apical system. The special lines in which 

 these modifications take place produce, of course, all 

 possible combinations, yet they give us the key to the 

 sudden appearance, as it were, of structural features of which 

 the relationship must be sought in very distantly related 

 groups. It is to this specialty in the paleontological devel- 

 opment that we must trace, for instance, the Cidarid affinities 

 of the Saleniac, their papillae, the existence of few large 

 primary interambulacral tubercles, the structure of their 

 apical system, and their large genital plates ; while it is to 



