ECHINOIDEA. II. 87 



typoidca, among which the ancestors of both Spatangoids, Cassidulids and Clypeastrids undoubtedly 

 must be sought for. The Holectypoidea again must be derived from the Diadcmina (or perhaps from the 

 Echinothurids (Strcptosomata)\ as must be concluded alone from their perforate and crenulate tubercles. 



Gregory (Op. cit.) divides the Atelostomata into the two suborders Asternata (Echinoncida, 

 Xnclcolitida- and Cassiditlidce) and Stcrnata (Collyritida;, Echinocorythidcr, Spatangidee, Palcpostomatida- 

 and Ponrlnlexiid(r). To this must be objected apart from the position of the I^ourtalesiida 

 that the CollyritidcF are really asternous. Since the Collyritida' evidently cannot be referred to his sub- 

 order Asternata, their relation being decidedly with the Spatangoids, I think we must let them rank 

 as a distinct suborder besides the Amphisternata and Mcridosterna ta ; I propose to name this suborder 

 Protosternata. 



In my view the ancestral history of the Irregular Echinoids may then shortly be comprised as 

 follows. The Holectypoidea, which are derived from the Diademina, develop into three separate main 

 groups: the Clypeasfroidca, Cassiduloidea and Spatangoidea. In the former the masticatory apparatus 

 undergoes a further development, in the two latter groups it becomes lost. Leaving out of considera- 

 tion the Clypeastroidea and Cassiduloidea we ma)- follow the third branch, the Spatangoidea. From 

 the more primitive forms of this group, represented by the Collyritida, two separate main branches 

 have developed ', each characterized by their peculiar structure of the plastron, in one meridosternous, 

 in the other amphisternous. The Meridosternata develop through the Ananchytid<e , of which the genus 

 Stercopneustes is the only known living representative, into three separate branches, the Urechinidce, the 

 Calymnid<F and the Pourtalesiidce. The Amphisternata I cannot here follow in a more detailed manner, 

 having not yet had occasion to study them all very closely; but I think it beyond doubt that the 

 more primitive forms are those included by Lambert and Agassiz in the families Acropidce and 

 Palcpopneustida", together with the Pal(eostomatid<r, the more specialised forms being such as Spatan- 

 gus, Brissus etc. 



To seek for transitional forms between the Pourtalesiae and the more primitive amphisternous 

 forms is, so far as I can see, rather absurd. The Pourtalesise are so far from being embryonic Spa- 

 tangoids* 2 that they must be regarded as the' most specialized branch of the whole group, in which 

 the development has been carried out to such extremes that it may be hard enough to see the 

 accordance with the general rules of the echinoid structure. In the Cltallenger-Echinoidea (p. 130) 

 Agassiz finds the affinities developed in so many directions in the group of Pourtalesise (is) one of 

 its most interesting features*, tracing its relationship to the Brissina, and to such genera as Hcmi- 

 aster, Echinocardmm, Lovenia and the like through Aerope, Aceste and Cionobrissus^ , further to the 

 Spatangiua proper through such genera as Pakeotropiis, Genicopatagus and Homolampas, and again 

 to the Galeritidte and Echinolampadse through such genera as Urechinus and Cystechimis* , besides 



sthe many-sided affinities to the Ananchytidse, Dysasteridse, and such genera as Cardiaster, Ho- 



laster, Toxastcr and the like?. Also to the Clypeastroids the Pourtalesise are said to show affinities, 

 viz. in the simple actinostome and in the structure of some of the pedicellariae (Op. cit. p. 129. Note), 



1 I do not mean to say that the}- have developed directly from the Collyritida; the real ancestor of the Merido- 

 slcrnata and Amphislernaia must have had a simple, not disconnected apical system. 



2 Rev. of Kch. p. 347. The expression is, strictly speaking, used only of In/utaster and the Ananrliylirlcr. 



