and Embryological Development. 357 



sequence of structural characters uniting the successive genera 

 of Holasteridge, such as Cardiaster, Offaster, Stenonia, Anan- 

 chytes, and Asterostoma, with Palaiojmeustes, Homolampas, 

 and the Pourtalesice of the present day, while from the genera 

 of the Toxasteridse we naturally pass to the Cretaceous Hemi- 

 aster; in this genus and the subsequent Micraster we find all 

 the elements necessary for the modifications which appear in 

 the Spatanginge 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 ab- 

 actinal 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 modifications of the ambulacral system in Spa- 

 tangus, Agassizia, and Echinocardium, often recalling in some 

 of its features structural characters of families which have pre- 

 ceded this in time. 



Apparently in striking contrast with the Echini of the 

 Secondary period and those which have succeeded them stand 

 the Palgeozoic Echini ; but when we have examined the 

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

 stand the structure and the affinities of the Palfechinidge 

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

 decessors. 



Taking up now the embryological development of the 

 several families which will form the basis of our comparisons, 

 beginning with the Cidaridge, 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 Diadematidge 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 Arbaciadge the young stages are remarkable for the 

 prominent sculpture 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 genital 

 ring. 



In the Echinometrada3 the young thus far observed are 

 characterized by the small number of their primary tubercles, 

 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- 



