scHurnKRT.l SUMMARY OP BIOLOGIC CHARACTERS. 103 



Carboniferous systems. One hundred and twenty-one American spe- 

 cies are also found on other continents. 



Widely dispersed species are least common in the most primitive 

 order, Atremata, and greatest in the highest orders, Protremata and 

 Telotremata. The diflterence, however, is but 7 per cent. 



The order Atremata is represented by 199 species, or over 10 i)er 

 cent of the American Paleozoic representation. In the TSTeotremata it 

 is 15(1, or over 8 per cent. The Protremata have 738 species, or nearly 

 40 i^er cent; And the Telotremata 766 species, or about 41 per cent. 



The order Atremata is best developed in species and genera in the 

 Cambrian and Ordovician systems; the ISTeotremata in tlieOrdovician; 

 the Protremata in the Ordovician, Silurian, and Devonian; and the 

 Teh)tremata in the Devonian. The climax of differentiation is there- 

 fore chronologically related to phylogenetic or sequential origiu. 

  Since the four orders of Brachiopoda are present in the Lower Cam- 

 brian, ordinal differentiation must have taken place in pre-Cambrian 

 times. The two more primitive orders, Atremata and Neotrematti., 

 have in Lingula and Crania, respectively, genera with longest life 

 histories. This probably is due not so much to their primitive struc- 

 tures as to their modes of living. 



The last order to originate, Telotremata, has the greatest number of 

 generic and superfamily characters, and probably also of sjiecies. 



The last sui^erfamily to appear, Spiriferacea, manifests most rapid 

 evolution and is the second one to die out, being preceded by the Pen- 

 tameracea. These two superfamilies are the most highly speciali;ced in 

 the orders to which they belong, and their great specialization may be 

 the cause of their early disappearance. 



The trunk families of later origin throughout the class manifest the 

 greatest specific and generic differentiation and the widest specific dis- 

 persion, and have species of the largest size and often of longer geologic 

 persistence. • 



The oldest or most primitive families nearly always have short geo- 

 logic duration (except Ehynchonellidjv) and the least generic and spe- 

 cific differentiation, and commonly the individuals are of small size. 



The largest of all brachiopods occur in the families Peutameridpe, 

 Productid;e, and Spiriferidte, at a time when the class was at the height 

 of differentiation. 



Large specific size is probably often gradually attained in genetic 

 lines, and is due to favorable food conditions. The gigantic brachio- 

 pods always occur in the later-developed trunk families, and just before 

 their decline in differentiation. 



But 8 genera are known to pass from the Paleozoic to the Mesozoic. 

 There are in all 327 brachiopod genera, 227 of which are Paleozoic. 

 The Atremata have 29 genera, the E'eotremata 30, the Protremata 89, 

 and the Telotremata 179. 



All brachiopods begin with smooth shells and protegula. 



