82 SYNOJ'SIS OF AMERICAN FOSSIL BKACHIOPODA. [bull. 87. 



the Ordovician there are 20 genera and 173 .si)ecies, a specific, increase 

 of more than seven and one-lialf times the nnniber in the Canil^rian. 

 Greatest generic dillcrentiation occnrred dnring the Silurian, wliere 'SO 

 genera appear. Then began a steady decline, with extinction in the 

 Carboniferous of North America. In tlie Triassic of Europe this order 

 is sparingly represented by small species, and is there essentially 

 restricted to the family Thecidiidtc, which continues to have living 

 representatives in the Mediterranean Sea. 



The widely distributed species gradually increase in percentage from 

 14 in the Cambrian to 3G in the Carboniferous, and are most marked 

 in the family Productidte. This family is one of the last of the order 

 to originate. 



The largest of all brachiopods occur in this order, in the families 

 Pentameridfe and Productidie, exceeding the Spiriferid* of the Telo- 

 tremata. In the former family greatest size is attained in the Silurian 

 during the acme of the order, and in the Productida^ in the Carbonif- 

 erous system. Productus giganteus of the Lower Carboniferous is the 

 giant of all brachiopods, attaining a diameter of nearly 1 foot. In 

 both these families the earliest species are sumll, but certain groups 

 gradually attain larger and larger size with geologic time. Upon the 

 appearance of the giants, vitality of the families, as exemplified in 

 specific differentiation and robustness of individuals, is at its highest. 

 After this these families rapidly decline, and the species dwarf far 

 more rapidly than they developed to the climax. 



In the Protremata, as in the two previous orders, greatest specific 

 differentiation does not occur in the radical families, but in those of 

 later development. The Kutorginida', Clitambonitidie, and Billingsell- 

 idse are the radical and, geologically, the oldest families of the Pro- 

 tremata. These are best but sparingly developed in the Cambrian, 

 whereas the younger families, Pentameridas Strophomenidse, Produc- 

 tidie, and Orthidic, contain over 95 per cent of the species and nearly 

 90 per cent of the genera. Orthidte and Strophomenidte, beginning 

 in the Cambrian, are best developed in the Ordovician and Silurian 

 systems, respectively; while Productid*, originating in the Silurian, 

 attained a climax in the Carboniferous. The latter family was one of 

 the last of the Protremata to originate and has the shortest geologic 

 history and least generic differentiation, yet many of its species have 

 greater geographic dispersion. 



The Protremata are clearly divisible into two phyla, Strophomenacea 

 and Pentameracea. The former superfamily has the greater number of 

 species, and is characterized by the nondevelopment of a spondylium or 

 cruralium. The Pentameracea has, in addition to the deltidium, an inter- 

 nal spoon-shaped plate, or spondylium, serving for the attachment of 

 muscles, and a discrete or united cruralium. The superfamily Stropho- 

 menacea in North America has (308 species, and represents the most 

 primitive phylum, since it is far better developed in the Cambrian than 



