scHirnERT] SUMMARY OF BIOLOGIC CHARACTERS. 103 



Ciiiboniterous sj-steius. Oiio huiuUed and twenty-ouc American spe- 

 cies are also found on other continents. 



Widely dis[)erscd s[)ecies are least common in the most primitive 

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

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



The order Atremata is represented by 199 species, or over 10 per 

 cent of the ^Vmerican Paleozoic representation. In the Neotremata it 

 is lo(i, or over 8 per cent. The Protremata have 738 s[)ccies, or nearly 

 10 pel- cent; and the Telotremata 7()G species, or about 41 ])er cent. 



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

 Cambrian and Ordovician systems; the Neotremata in the Ordoviciaii; 

 the I'rotremata in tlie Ordovician, Silurian, and Devonian; and the 

 Telotremata in the Devonian. The climax of differentiation is there- 

 fore chronologically related to phylogeuetic or sequential origin. 



Since the four orders of ]>rachiopoda 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 mu(;h to their i)rimitive 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 species. 



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 specialized 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- 

 l^ersion, and have species of the largest size and often of longer geologic 

 j)ersistence. 



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

 logic duration (except Rhynchonellida') and the least generic and spe- 

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



The largest of all brachiopods occur in the families Pentamerid^e, 

 Productidic, and Spiriferidse, 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 Neotremata 30, the Protremata 89, 

 and the Telotremata 1 79. 



All brachiopods begin with smooth shells and protegula. 



