ECHINOIDEA. II. 87 



typoidca, aniouy which the ancestors of botli Spatanooids, Cassidulids and Cl\peastrids undoubtedly 

 must be sought for. Tlie HolectNjJoidea again nuisl be deri\ed from the Diadcmiiui (or perliaps from the 

 Echinothurids 1 Stnptosoinafn}], as must be conchided alone from their perforate and crenulate tubercles. 



Gregory (Op. cit.) (li\'ides the ^[/(-/os/oj/Ki/d into tlie two suborders Aslrriiatd (Echinoncidff, 

 Xiiclralifida- and Cassidiilidtr) and S/rriKita { Collyritidcr, J'lcliiiiocorythidcr, Spatangida', Palcp.ostomatidcr 

 and I'oitrtalcsiidicj. To tliis must be objected — apart from the position of tlie Potirtalesiidw — 

 that the CollyrUidcr are realh' asternous. Since the Collyritidce. evidently cannot be referred to his sub- 

 order Astcniatu, their relation being decidedJN with the Spatangoids, I think we must let them rank 

 as a distinct suborder besides the ^[iiiphistrnuild and Mcridostrrmila ; I ]5ropose to name this suborder 

 Protosternata. 



In niv view the ancestral history of the Irregular Echinoids ma\- then shortly be comprised as 

 follows. The Uolcctypoidca. which are derived from the Dii/di'iiiiiKi, de\-elop into three separate main 

 groups: the Clyprastfoidra, Cassidtdoidea and Spdidiigoidid. In the former the masticatory apparatus 

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

 tion the Clypeastroidca and Cassiduloidea we may follow the third branch, the Spatangoidca. From 

 the more primiti\e forms of this group, represented Ijy the Collyritida'. two separate main branches 

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

 in the other amphisternou.s. The A/rn'dosfrninhi develop through the Aiuuichytidcr. of which the genus 

 Strrcopneustes is the only known living representative, into three separate branches, the Urcchinidcc. the 

 Calyiiniido' and the Pourtalesiidcr. The Amphistmuita 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 liy Lambert and Agassiz in the families Aeropida; and 

 /'(rla-opi/CHstid/r. together with the PalirosfoiiKifidif. the more specialised forms being such as Spataii- 

 g/is, Brissus etc. 



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

 forms is, so far as I can see, rather absurd. The Pourtalesi:e are so far from being ,embr\onic Spa- 

 tangoids ^ that the\' 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 ma\' be hard enough to see the 

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

 Agassiz finds the affinities developed in so many directions in the group of Pourtale.sise (is) one of 

 its most interesting features , tracing its relationship <'to the Brissina, and to such genera as Ilniii- 

 astcr, EcIunocardiKin. Lovciiia and the like through Ai'ropr. Accste and Cioiwbrissiis . further < to the 

 Spatangina proper through such genera as Pahcotropiis, Goiicopatagiis and lIcDiioltiDipas. and again 

 to the Galeritidie and Echiuolampadse through such genera as Urccliiints and Cys/ic/iimis^, besides 



the manv-sided affinities to the Ananclntidce, Dysasteridae, and such genera as Cardiastcr, llo- 



lastrr. Toxastrr and the like . Also to the Cl\-peastroids the Pourtalesiie are said to show affinities, 

 viz. an the simple actinostome and in the structure of some of the pcdicellaritc (Op. cit. j). 129. Note), 



■ I do not mean to sa\' that the-\- liave developed directh- from tlie Collyiilidti-: the real ancestor of the Mericio- 

 s/i-rna/n and Amphisteynata must have had a simple, not disconnected apical system. 



2 Rev. of EX'h. p. 347. The expression is, strictly speaking, used only of In/ulaslci- aud the Auaiirliytid(r. 



