100 HAWAIIAN AND OTHER PACIFIC ECHINI. 



following grouping of the families and genera of Spatangina does but blaze the 

 way towards a natural classification of the group, its purpose is accomplished. 

 I can only hope it is not headed in the wrong direction. 



Key to the Families of Spatangina. 



Interambulacrum 5 not essentially different orally from the other interambulacra (Asternata) . 

 Ambulacra not at all petaloid; peristome oblique; periproct on oral surface 



close to mouth Echixoxeidae 



Ambulacra subpetaloid; peristome not oblique; periproct near or above 



margin Nucleolitidae 



Interambulacrum 5 modified orally to form a sternum. 

 Labrum followed by a single plate (Meridosternata) . 

 Mouth horizontally placed on oral surface of test. 



Interambulacra all meridoplacous Urechixidae 



Interambulacra 2 and 3 amphiplacous. • 



Ambulacral pores in pairs Echixocortthidae 



Ambulacral pores simple . . ' Calymxidae 



Mouth vertical at the end of an oral furrow or invagination .... Pottrtalesudae 

 Labrum followed by a pair of nearly, or quite, equal, large plates (Amphi- 

 sternata) . 



Peristome approximately circular or pentagonal with unmodified inter- 

 ambulacral plates around it and with no indication of a labrum (except 

 in Aceste); no petals; peripetalous fasciole present. 



Peristome pentagonal, closed by 5 equal, triangular plates . . . Palaeostomatidae 

 Peristome circular covered by a membrane, either naked or with few, 



scattered, small plates Aeropsidae 



Peristome more or less transversely elongated, with the primordial plate 

 of interambulacrum 5 modified to form a more or less well-marked 

 labrum. 

 Ambulacra flush, not petaloid or imperfectly so; fascioles wanting, 



or subanal and, in a few species, peripetalous, present .... Palaeopxetstidae 

 Some or all of the ambulacra more or less sunken, or more or less 

 petaloid, or both. 



Subanal fasciole wanting Hemiasteridae 



Subanal fasciole present Spatangidae 



ECHINONEIDAE Wrigfct. 



This family includes but few Recent species. It is, however, very dis- 

 tinctly differentiated and its relationship to other Echini is quite obscure. 

 Hawkins (1912. Proc. Zool. Soc. London, p. 400) considers the family most 

 nearly allied to the Conulidae, having arisen with that family from some 

 Pygaster-like form, and no better suggestion has been made. The discovery 





