NOVEIMBEE29, 1895.] 



SCIENCE. 



723 



Specific dififerentiation was most rapid in 

 the Ordovician, having exceeded the Cam- 

 brian representation more than three times. 



Thirty per cent, of all American Paleo- 

 zoic species had wide geographic distribu- 

 tion, and this is most pronounced in the 

 Devonian and Carboniferous systems. One 

 hundred and twenty-one American species 

 are also found on other continents. 



Widely dispersed species are least com- 

 mon in the most primitive order, Atremata, 

 and greatest in the highest orders, Protre- 

 mata and Telotremata. The difference, 

 however, is but seven per cent. 



The order Atremata is represented by one 

 hundred and ninetj^-six species, or over ten 

 per cent, of the American Paleozoic repre- 

 sentation. In the IS'eotremata it is one 

 hundred and fifty-three, or over eight per 

 cent. The Protremata have seven hundred 

 and thirty-five species, or nearly forty per 

 cent., and the Telotremata seven hundred 

 and sixty-two species, or about forty-one 

 per cent. 



The order Atremata is best developed in 

 species and genera in the Cambrian and 

 Ordovician systems ; the Neotremata in the 

 Ordovician ; the Protremata in the Ordo- 

 vician, Silurian and Devonian; and the 

 Telotremata in the Devonian. The climax 

 of differentiation is therefore chronologi- 

 cally related to phylogenetic or sequential 

 origin. 



Since the four orders of Brachiopoda are 

 present in the Lower Cambrian, ordinal 

 differentiation must have taken place in 

 Pre-Cambrian times. The two more primi- 

 tive orders, Atremata and Neotremata, have 

 in lAngula 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 

 species. 



The last superfamily to appear, Spirifer- 

 acea, manifests most rapid evolution and is 

 the second one to die out, being preceded 

 by the Pentameracea. These two superfam- 

 ilies 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 through- 

 out the class manifest the greatest specific 

 and generic differentiation, the widest spe- 

 cific dispersion, and have species of the 

 largest size and often of longer geologic 

 persistence. 



The oldest or most primitive families 

 nearly always have short geologic duration 

 (except Ehynchonellidce) , the least generic 

 and specific differentiation, and commonly 

 the individuals are of small size. 



The largest of all brachiopods occur in 

 the families Pentamerida', Productida, and 

 Spiriferidce, at a time when the class was at 

 the height of differentiation. 



Large specific size is probably often gradu- 

 ally attained in genetic lines, and is due to 

 favorable food conditions. The gigantic 

 brachiopods always occu^r in the later de- 

 veloped trunk families, and just before their 

 decline in differentiation. 



But eight genera are known to pass from 

 the Paleozoic to the Mesozoic. There are 

 in all three hundred and twenty-one bi-achio- 

 pod genera, two hundred and twenty-three 

 of which are Paleozoic. The Atremata 

 have twenty- six genera ; the Neotremata, 

 thirty-one ; the Protremata, eighty-seven ; 

 and the Telotremata, one hundred and 

 seventy-five. 



All braciiiopods begin with smooth shells 

 and protegnla. 



The prodeltidium, or third embryonic 

 shell plate, is known in the Atremata, 

 Neotremata and Protremata. In the Atre- 

 mata this becomes attached to the dorsal 

 valve, while in the Telotremata it is not 

 apparently developed at all. In the Pro- 



