144 



SCIENCE. 



tubercles, and an Echinid poriferous zone. In the same 

 way in the Diadematidse, the large primary internambulac- 

 ral 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 modi- 

 fied in the same direction as that of the Echinidae. 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 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 gen- 

 era following them in time, to be explained at present only 

 on the supposition that, when a structural feature has once 

 made its appearanc, it may reappear subsequently, appa- 

 rently as a new creation, while in reality it is only its pecu- 

 liar 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 care- 

 ful analysis, not only of the genera of the order, but some- 

 times 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 fea- 

 tures 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 seem 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 exist- 

 ence of the genus Pygaster or Cidaris 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, etc.). 



Passing to the Clypeastridae, 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 embryoiogical grounds, the modifi- 

 cations the earliest Desmosticha-like Echini should under- 

 go in order to assume the structural features of Pygaster, 

 yet the early periods in which the precursors of the Echino- 

 conidae and Clypeastridae are found have thus far not pro- 

 duced 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 Echinanthidae, 

 Laganidae, and Scutellidse, 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 pass from Pygaster to Holectypus, to 

 Echinocyamus, and Echinanthidae, we should have in the 

 genera which follow each other in the paleontological 

 record an unbroken scries showing exactly what these 

 modifications have been. In the same way, the modifica- 

 tions of the abactinal and anal systems, and those of the 

 poriferous zone, can equally well be followed to Echino- 

 cyamus, and thence to the Clypeastridae ; while a similar 

 sequence in th ■ modifioations of these structural features 

 can be followed from Mortonia to the Scutellidse of the 

 present period. 



Passing finally to the Petalosticha, we find no difficulty 

 in tracing theoretically the modifications which our early 

 Echinoconkbe of the Lias should primarily undergo previous 

 to the appearance of GaleropygUS. The similarity of the 

 early Cassiduloid and Echinoneoid types points to the 

 same systematic affinity, and perhaps even to a direct and 

 not very distant relationship with the Palaechinidae. For if 

 we anal ze the Echinoihuri;e of the present day, we find in 

 genera like Phormosoma many structural features, such as 

 the shape of the test the character of the spines, the structure 

 of the apical System, that of the poriferous zone, indicative 

 of possible modifications in the direction of Pygaster or of 

 GaleropygUS, which have as yet not been taken into 

 account. 



Adopting for the Petalosticha the same method cf trac- 

 ing the modifications of single structural features in their 

 paleontological succession, we trace the comparatively 

 little modified paleontological history of the Echinoneidse 

 of the present day from the Pyrina of the lower Jura. This, 

 in its turn, has been preceded by Hyboclypus and Galero- 

 pygUS, while the Echinolampadae of the present day date 

 back, with but trifling modifications, to the Echinobrissus 

 of the Lias, itself preceded by Clypeus ; and they have 

 been subject only to slight generic changes since that time, 

 Echinobrissus being still extant, while such closely allied 

 genera as Catopygus and Cassidulus of the earlier Creta- 

 ceous are still represented at the present day ; the modifi- 

 cations taking place in the actinal system, in the ambulacal 

 zones of the Echinoconidae and of the Echinolampadae 

 showing the closest possible systematic affinity in these 

 families. Starting again from Hyboclypus, with its elongate 

 apical system, we naturally pass to Collyrites and the 

 strange Dysasteridae forms which, in their turn, are closely 

 allied to the Holasteridae. $ From Holaster on the one side, 

 and from Toxaster on the other, we find an unbroken 

 sequence of structural characters uniting the successive 

 genera of Holasteridae, such as Cardiaster, Offaster, Stenonia, 

 Ananchytes, and Asterostoma, with Paleopneustes, Homo- 

 lampas and the Pourtalesiae of the present day, while from 

 the genera of the Toxasteridae we naturally pass to the 

 cretaceous Hemiaster ; in the genus and the subsequent 

 Micraster we find all the elements necessary for the modi- 

 fications which appear in the Spantanginae from the time of 

 the Chalk to the present day. These modifications result 

 in genera in which we trace the development of the fascioles, 

 of the actinal, anal, and abactinal plastrons, of the beak, 

 the formation of the petaloid ambulacra, first flush with the 

 test, and little by little changed into marsupial pouches, 

 the growth of the anterior groove and the manifold modifi- 

 cations of the ambulacral system in Spatangus Agassizia, 

 and Echinocardium, often recalling in some of its features 

 structural characters of families which l:ave preceded this 

 in time. 



Apparently in striking contrast with the Echini of the 

 secondary period and those which have succeeded them 

 stand the Paleozic Echini ; but when we have examined the 

 embryology of Echini, we shall be better prepared to under- 

 stand their structure and the affinities of the Palaechinidae 

 with the Echini of the present day and their immediate pre- 

 decessors. 



Taking up now the embryoiogical development of the 

 several families which will form the basis of our compari- 

 sons, beginning with the Cidaridae, we find that in the 

 earliest stages they very soon assume the characters of the 

 adult, the changes being limited to the development of the 

 abactinal system, the increase in number of the coronal 

 plates, and the modifications of the proportionally gigantic 

 primary radioles. 



In the Diadematidae the changes undergone by the young 

 are limited to the gradual transformation of the embryonic 

 spines into those which characterize the family, to the 

 changes of the vertical row of pores in the ambulacral 

 area into arcs of three or four pairs of pores, and to the 

 specialization of the actinal and abactinal systems. 



In the Arbaciadac the young stages are remarkable for the 

 prominent sculplure of the test, for the flattened spines, for 

 their simple poriferous zone, for their actinal system, and for 

 their genital ring.. The anal plates appear before the geni- 

 tal ring. 



In the Echinometradac the young thus far observed are 

 characterized by the small number of their primary tuber- 

 cles, the large size of the spines, the simple vertical row of 

 pores, the closing of the anal ring by a single plate, and the 

 turban-shaped outline of the test. Little by little, the test 

 loses with increasing age this Cidaris-like character ; it re- 

 minds us, from the increase in the number of its plates, 

 more of Ilemicidaris ; then, with their still greater increase, 

 of the Pscudodiadematidac; and, finally, of the Echinome- 

 trad.e proper. The spines, following pari passu the changes 

 of the test, lose little by little their fantastic embryonic, or 

 rather Cidaris-like appearance, and become more solid and 

 shorter, till they finally assume the delicately fluted structure 

 characteristic of the Echinometradae. The vertical porife- 

 rous zone is first changed into a series of connected verti- 

 cal arcs, which become disjointed, and form, with increas- 



