2G2 Sohuohert — Jackson on the Phytogeny of the Echini. 



are non-existent. Mr. Agassiz (1909) has shown that in the 

 young of the spatangoid Echinoneus a well developed lantern 

 exists. This discovery is of the greatest interest and impor- 

 tance, as previously teeth were unknown in this group. . . . 

 Looking back to the Centrechinoida, we find that this type of 

 lantern exists only in the suborder Stirodonta. Further, the 

 attachment of muscles, as stated, occurs only in Arbacia and prob- 

 ably other members of its family. ... I therefore consider the 

 Exocycloida as connected with the Arbaciidae, probably through 

 some early common ancestral stock " (217-8). The Exocy- 

 cloida have 3 suborders : Hol.ectypina (Jurassic to Eocene) 

 with the ambulacral plates compound or simple and with the 

 ambulacral areas not petaloid dorsally ; Clypeastrina (Creta- 

 ceous to Recent) with more or less flattened tests, ambulacral 

 plates simple and the areas petaloid dorsally, while the lantern 

 is highly modified ; Spatangina (Jurassic to Recent) with the 

 ambulacral plates simple and the areas commonly petaloid dor- 

 sally but with no lantern nor perignathic girdle in the adults. 



Order Piesiocidaroida, an aberrant and imperfectly known 

 stock restricted to the Triassic (Tiarechinus), in which the peri- 

 proct is central but the genitals are large and occupy most of 

 the dorsal surface. There are 2 columns of ambulacrals and 

 3 of interambulacrals. Plates not imbricate. Base of corona 

 not resorbed. " It is not closely affiliated with any other 

 group " (220). 



Order Perischoechinoida, arose in the Bothriocidaroida at 

 least as early as the Silurian and persisted into the Permian. 

 Corona and periproct regular in form and position, with from 

 2 to 20 columns of plates in each ambulacral area and from 3 to 

 14 in each interambulacral area. No perignathic girdle, the 

 lantern muscles attaching directly to the base of the interam- 

 bulacral plates. Embraces the families 



Archaeocidaridoz (Eocidaris, Archaeocidaris, Lepidocidaris), 

 with 2 cohxmns of ambulacrals and 4 to 8 of interambulacrals, 

 plates thin and imbricating, base of corona resorbed, and pri- 

 mary spines large ; Devonian to Permian. 



Lepidocentridcm (Koninckocidaris, Lepidocentrus, Hyatte- 

 chinus, Pholidechinus), with 2 columns of ambulacrals and 5 

 to 14 of interambulacrals, plates thin and imbricating, base of 

 corona not resorbed, and all of the spines small ; Silurian to 

 Mississippian. 



Pcdacechinidce or Melonitidse (Palaeechinus, Maecoya, Loven- 

 echinus, Oligoporus, Melonechinus), with 2 to 12 columns of 

 ambulacrals and 3 to 11 of interambulacrals, plates not imbri- 

 cate, some resorption of base of corona, only small secondary 

 spines ; Silurian to Mississippian. 



In this family the genealogical relations of the genera are 



