252 Schuchert — Jackson on the Phytogeny of the Echini. 



chinus, Meekechinus, and Ph olid echinus). Of good species 

 there are 119 and of these 23 are new. Of incertae sedis and 

 nomina nuda there are 3 genera and 34 named forms. 



Ordovician. — The oldest and most primitive Echini occur 

 in the Middle Ordovician of Esthonia, Russia, where the genus 

 Bothrioeidaris is found with 3 exceedingly rare species (1 in 

 the J ewe and 2 in the Lyckholin formations). 



Silurian.- — The oldest American representative of the class 

 was recently found in the Rochester shale of New York 

 {Koninckocidaris silurica, n. sp.). In the Llandovery of Eng- 

 land occurs Maccoya phillipsiae, while the lower Ludlow has 

 furnished Palaeodiscus ferox and Eehinocystites pomum. 



Devonian. — Germany has in the Middle and Upper Devon- 

 ian Xenocidaris (3 species), Eocidaris laevispina, and Lepido- 

 centrus (3). In the Upper Devonian of New York is found 

 L. drydenensis. In England occur Lepidesthes devonicans 

 and P holidocidaris acuaria. 



Lower Carboniferous. — The Lower Carboniferous is the 

 period of greatest development of Paleozoic Echini and in 

 America alone there are 50 good species, with 31 more in 

 Europe. In the Millsap formation of Colorado occurs Mio- 

 cidaris cannoni, a new species and the oldest stratigraphically 

 of the Cidaridag, the stock that gave rise to Mesozoic and later 

 Echini. Archaeocidaris has 10 American species and 12 other 

 forms occur in Europe. Other genera in America are Lepido- 

 cidaris (1 species), Lepidocentrus (1), Hyattechinus(3), Pholide- 

 chinus (1), Palaeechinus (1), Maccoya (2), Lovenechinus (4), 

 Oligoporus (5), Melonechinus (11), Lepidechinus (3), Perischo- 

 donius (1), Lepidesthes (6), Pholidocidaris (1). 



Upper Carboniferous. — "In the Upper Carboniferous the 

 Palaeozoic Echini have dropped out with extreme suddenness 

 and relatively few species are known." Archaeocidaris has 17 

 species in A.merica and 2 in Europe. The only other form is 

 the American Lepidesthes extremis, n. sp. 



Permian. — The cidaroid Miocidaris keyserlingi occurs in 

 Germany and England. Of Archaeocidaris there is 1 species 

 in America, 1 in India, and 1 in Australia. The only other 

 form is the American Meekechinus elegans, n. sp. 



Types of variation defined. 



Jackson states that next to stages in development variation 

 is an extremely important subject as a basis in phylogenetic 

 determinations. Echini are especially valuable on which to 

 study variation, because in them variation can be so definitely 

 expressed. It is seen in the introduction of columns, number 

 of plates in a row, number of oculars that reach the periproct, 

 etc. 



