ECHINOIDEA. II. 



87 



typoidca. among which tlie ancestors of both Spatangoids, Cassidulids and Clypeastrids undonbtedly 

 must be sought for. The Holectypoidea agaiii must be derived from the Diadcinina (or perhaps from the 

 Echiiiothurids (SfrcpfosoDinfoj), as mxist be conchided alone from their perforate and creniilate tubercles. 



Gregory (Op. cit.) divides tlie Afclostomata into the two suborders Astrriiata {EchiiioiiiidcF, 

 NiiclrolHida- and Cassidiilida-) and Slrniata (CollyrHidæ, Kcliiiiocorytliidif. Spalangid(r. Palæostoiiiatidæ 

 and I'oiirtdlrsiido-). To this must be objected — apart from the position of the I'ourtdlrsiidcr — 

 that the CnllyrHidtT are really asternous. Since the Collyritidcr evidentl_v cannot be referred to his sub- 

 order Astcniatti, their relation being decidedh' witli the »Spatangoids, I think \ve nnist let them rank 

 as a distinct stdjorder besides the ^[inphistcnutta and Mrridnstrniafd ; I propose to name this suljorder 

 Protosternata. 



In my view the ancestral history of the Irregular Echinoids ma\' tlicn shortly be comprised as 

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

 groups: the Clypcastroidra, Cassidtiloidea and Spafangoidra. In the former the masticatory apparatus 

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

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

 the more primiti\e forms of this gronp, represented by the Collyritidcr, two separate main branches 

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

 in the other amphisternou.s. The iMrridostcriiata develop tlirough the ^\naiicliytidæ, of which the genus 

 Stcreopncitstfs is the only known living representative, into three separate branches, the Urcchinidir, the 

 Calyiimidu- and the Ponrtalesiidæ. The Amp/iistcriiatn 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 douI:)t that the 

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

 Palæopnenstidæ, together with the Palæostomatidæ , the more .specialised forms being sucli as Spafan- 

 gus, Brissiis etc. 



To seek for transitional forms between the Pourtalesiæ and the more primiti\-e amphisternous 

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

 tangoids > ^ 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 ma\- be hard enough to see the 

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

 Agassiz finds the atfinities developed in so many directions in the group of Pourtalesiæ (is) one of 

 its most interesting features , tracing its relationship to the Brissina, and to such genera as Hrii/i- 

 astcr, Ecliiiiocardiuni. Lovenia and the like through ^Icropr. Accste and Ciorwbrissus , further »to the 

 Spatangina proper through such genera as Palæotropns, Gcnicopatagtis and Hoiiiolatiipas, and again 

 to the Galeritidæ and Kchinolampadæ through such genera as Ureelt i mis and Cysteeliii/iisu besides 



«the many-sided affinities to the Ananchytidæ, Dysasteridæ, and such genera as Cardiaster, ffo- 



laster. Toxaster and the like . Also to the Clypeastroids the Pourtalesiæ are said to show affinities, 

 viz. ^ in the simple actinostonie and in the structure of some of the pedicellariæ (Op. cit. p. 129. Note), 



> I (lo not meau to say that they have developed directly from the Collyritida:; the real ancestor of the Mfrido- 

 s/rrnnln and Ainphi.slernata innst have had a simple, not disconnecteil apical .system. 



2 Rev. of Kell. p. 347. The expression is, strictly speakinj<, used only of hi/ii/nsler and Ihe Anaiirliylidir. 



