488 THE CRETACEOUS PERIOD 



Arthropoda. — Among the Crustacea we need only note the 

 great increase in the Brachyuran Decapods, or Crabs. 



Brachiopoda are very much as in the Jurassic ; the common 

 genera are Terebratula (X, 4), Terebratella (X, 5), and Rhyncho- 

 nella. 



Mollusca. — This group is very richly developed and many 

 genera are peculiar to the period. The large, curious oysters be- 

 longing to the genera Ostrea (X, 6), Gryphcza, and especially 

 Exogyra, are common, and the many species of Inocera?tius (X, 7) 

 are very characteristic. Confined to the Cretaceous are the ex- 

 traordinary shells classed as Rudistes, in which one valve is long 

 and horn- shaped, and the other a mere cover for it. These shells 

 of the genera Hippurites, Radw/i/es, and Coralliochama are much 

 commoner in Europe than in America. Other peculiar Cretaceous 

 Bivalves are Requienia, Caproiina (X, 8), and Aucella (X, 18), 

 the latter also Jurassic. The Gastropods (PL X, Figs. 9, 10, 11), 

 are very much as in the Jura, but in the latter part of the period 

 come in many genera which reach their fullest development in 

 Tertiary and recent times, such as Fusus, Murex, Voluta, Cyprcea, 

 and many others. 



The Cephalopods are very peculiar ; in addition to numerous 

 Ammonoid genera with closely coiled shells of normal type, such 

 as Hoplites, Schlaznbachia, Placenticei'as, we find very many 

 shells entirely or partially uncoiled, or rolled up in peculiar ways, 

 which give to the Cretaceous Cephalopod fauna a character all its 

 own. In Crioceras the shell is coiled in an open, flat spiral, the 

 whorls of which are not in contact. Ancylocei-as has a similar 

 open coil, followed by a long, straight portion, and recurved ter- 



Explanation OF Plate X, p. 489. i. Uintacrinus socialis, 1/12. (Clark). 

 2. Pseudodiadema texanum. (Clark.) 3. Toxaster texanus. (Conrad.) 4. Tere- 

 bratula Harlani, 3/4. (Whitfield.) 5. Terebratella plicata. (Whitfield.) 6. Ostrea 

 larva. (Whitfield.) 7. Inoceramus problematicus, 3/4. (Meek.) 8. Caprotina 

 bicornis, 1/3. (Meek.) 9. Fasciolaria buccinoides. (Meek.) 10. Anchura ameri- 

 cana. (Meek.) 11. Margarita nebrascensis. (Meek.) 12. Ptychoceras Mortoni, 

 3/4. (Meek.) 13. Scaphites nodosus, 1/2. 14. Baculites compressus, 1/2. 

 (Meek.) 15. Belemnitella americana, 1/2. (Whitfield.) 16. Nodosaria texana, 

 enlarged. (Conrad.) 17. Micrabacia americana, 3/1. (Meek.) 18. Aucella 

 Piochi. (Gabb.) 



