BCHINOIDEA. II. 87 



typoidea, among which the ancestors of both Spatangoids, Cassidulids and Clypeastrids undoubtidh 

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

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



Gregory (Op. cit.l divides the Atelostomata into the two suborders Asternata (Echinoneida, 

 Nucleolitida and Cassidulidce) and Sternata (Collyritida, Echinocory/l/id<e. Spatangidee, PalaostomatidcR 

 and Pourtalesiidce). To this must be objected — apart from the position of the Pourtalesiida. — 

 that the Collyritida are really asteruous. Since the Collyritidee 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 Ampkisternata and Meridosternata ; 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 Clypeastroidea, 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 may follow the third branch, the Spatangoidea. From 

 the more primitive forms of this group, represented by the Collyritidce. 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 Ananchytidce, of which the genus 

 Stcreopncustes is the only known living representative, into three separate branches, the Urechinida, the 

 Calymnidce and the Pourtalesiida. The Ampkisternata 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 Aeropida and 

 Paheopueitstidce, together with the PalmostomatidtB, the more specialised forms being such as Spaian- 

 gus, Brissns etc. 



To seek for transitional forms between the Pourtalesia; 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 thev 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 Challenger -Eehinoidea (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 Hemi- 

 aster. Echinocardium, Lovenia and the like through Aerope, Aceste and Cionohrissus . further «to the 

 Spatangina proper through such genera as Palaotropus, Genicopatagus and Homolampas, and again 

 to the Galeritidae and Echinolampacke through such genera as Urechinus and Cystechinus . besides 



the rrianv-sided affinities to the Ananchytidae, Dysasteridae, and such genera as Cardiaster, Ho- 



laster, Toxaster 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 pedicellarke (Op. cit. p. 129. Note), 



1 I do not mean to sav that they have developed directly from the Collyrilidcr; the real ancestor of the Merido- 

 sternata and Ampkisternata must have had a simple, not disconnected apical system. 



- Rev. of Kill. p. 347. The expression is, strictly speaking, used only of Infutaster and the Ananchytida. 



