382 THE ORDOVICIAN PERIOD 



merit in the Silurian and Devonian. Ostracoda (PI. II, Fig. 17) 

 and Phyllopoda undergo no marked change. That terrestrial ani- 

 mal life had already begun is demonstrated by the occurrence of 

 a Centipede. 



Brachiopoda. — These shells increase very largely in abundance 

 and variety, the genera with hinged calcareous shells (Articulata) 

 now gaining the upper hand and reducing the horny-shelled kinds 

 to comparative insignificance. The most important genera are : 

 Orthis (PI. II, Fig. 7), Orthisina, Leptcena, Strophomena (PI. 

 II, Fig. 9), Platys tropin a and Rhynchonella. 



Bryozoa. — This is a group which has as yet yielded no repre- 

 sentatives from the Cambrian, but appears in the Ordovician. The 

 genera differ little from those which live in the modern seas. 



Mollusca. — One of the most striking differences between the 

 Ordovician and the Cambrian is the great advance made by the 

 Molluscs in the former period. The Bivalves (Pelycypoda) are 

 larger, more numerous, and more like modern forms (see PI. II, 

 Fig. 10). The Gastropoda likewise increase notably in size and 

 in numbers, especially the spirally coiled shells like Murchisonia 

 (PL II, Fig. n) and Pleurotomaria ; but neither Bivalves nor 

 Gastropods had anything like the relative importance which they 

 possess in modern times. 



Much the most significant change in the Mollusca, however, is 

 the great expansion of the Cephalopoda, a few of which appear in 

 the uppermost Cambrian, but in the Ordovician have become one 

 of the predominant elements in the marine life of the times. 

 These forms are all Nautiloids, most nearly allied to the modern 

 pearly Nautilus, with chambered shells, divided internally by simple 



EXPLANATION OF Plate II, p. 383. 1. Brachiospongia digitata, 1/4. 2. Di- 

 cranograptus ramosus, 1/2. (Hall.) 3. Diplograptus pristis. (Ruedemann.) 

 4. Phyllograptus typus. (Hall.) 5. Dendrocrinus polydactylus, 1/2. (Meek.) 

 6. Agelacrinus cincinnatiensis, 2/1. (Meek.) 7. Orthis lynx. 8. Rhynchonella 

 capax. 9. Strophomena alternata. 10. Ambonychia radiata, 2/3. (Hall and Whit- 

 field.) 11. Murchisonia Milleri, 2/3. (Hall.) 12. Orthoceras Duseri, 1/2. (Hall 

 and Whitfield.) 13. Asaphus gigas, 1/3. (Hall.) 14. Calymene callicephala, 2/3. 

 (Meek.) 15. Triarthrus Becki, 2/3. (Hall.) 16. Trinucleus concentricus. (Hall.) 

 17. Leperditia fabulites. (Ulrich.) 



