146 STUDIES IN EVOLUTION 



The long list of genera in this family may be easily divided 

 into two sections, which are often recognized as of family rank. 



I. AsAPHiD^. — Including the genera and sub-genera 

 Asaphus Brongniart (= Cryptonymus Eichwald), Asaphellus 

 Callaway, Asaphiscus Meek, Barrandia McCoy, Basilicus 

 Salter, Bathyurellus Billings, Bathyuriscus Meek, Bathyurus 

 Billings, Bolbocephalus Whitfield, Brachyaspis Salter, Bron- 

 teopsis W. Thompson, Doliohometopus Angelin, Gerasaphes 

 Clarke, Holasaphus Matthew, Homalopecten Salter, Isotelus 

 DeKay, Megalaspides Brogger, Megalaspis Angelin, Niobe 

 Angelin, Ogygia Brongniart, Ogygiopsis Walcott, Phillipsi- 

 nella Nov4k, Platypeltis Callaway, Ptychopyge Angelin, and 

 Stygina Salter. 



This is a tolerably homogeneous group, although some of 

 the Cambrian forms have a sufSciently archaic expression 

 to make them seem a little out of place with genera of so 

 pronounced a family type as Asaphus, Niobe, Ptychopyge, 

 Megalaspis, and Isotelus. 



The elements of the glabella are generally quite obscure, 

 and even its limits cannot be clearl}' made out in late genera, 

 as Stygina and Asaphus. The segmental nature of the gla- 

 bella is clearly shown in Ogygia, Ogygiopsis, Homalopecten, 

 Asap)hellus, Bronteopsis, and Bathyuriscus. 



The elements of the pygidium are obscurely marked in 

 Brachyaspis and Isotelus. Phillip sinella is a very small 

 form, and probably the young of an Asaphus. Barrandia, 

 Homalopecten, and Stygina serve as transition genera to the 

 Illsenidse. 



II. iLLiENiD^. — Including the genera and sub-genera 

 Illcenus Dalman, u^glina Barrande (= Cyclopyge Corda), 

 Bumastus Murchison, Bysplanus Burmeister, Hctillcenus 

 Salter, Holocephalina Salter, Hydrolenus Salter, Illamopsis 

 Salter, Hloenurus Hall, Nileus Dalman, Octillcenus Salter, 

 Panderia Volborth, Psilocephalus Salter, Symphysurus Gold- 

 fuss, and Thaleops Conrad. 



The IllEenidse form a much more compact group than the 



