THE CAMBRIAN PERIOD. 299 



succeeding epochs. Crepicephalus texanus (Fig. 118, b) and Olenoides curticei 

 (Fig. 127, b) are interesting trilobites belonging to the middle fauna. Ptycho- 

 paria kingi (Fig. 127, c) and P. antiqua (Fig. 118, c) are species characteristic of 

 the middle stage, but the genus ranges both below and above. Agnostus inter- 

 strictus (Fig. 127, d) and A. obtusilobus (Fig. 118, d) represent the lowest type of 

 trilobites. Brachiopods, as before, were next to the trilobites in importance. 

 Frotorthis billingsi (Fig. 127, e) and Linnarssonia transversa (Fig. 127, g) are 

 distinguishing species. Acrotreta gemma (Fig. 121, c and d) and some other 

 species of the lower fauna lived through the middle and into the upper stage. 

 Harttia matthewi (Fig. 127, /) represents an interesting gastropod of the time. 

 Hyolithes, representing the pteropods, were common; other pteropods were 

 few. Plates of cystidians (Eocystites primozvus, Fig. 123) have been found, 

 but no complete forms. 



The fauna as here characterized is that which prevailed in the Atlantic prov- 

 ince. In the interior and western province, no very distinct Middle Cambrian 

 fauna has been detected and the life of that stage does not seem to have closely 

 corresponded to that of the Atlantic province. 



The Upper Cambrian or Dikellocephalus fauna. — The most characteristic trilo- 

 bite of the Upper Cambrian of America is the Dikellocephalus, while in Europe 

 it is Olenus, though Dikellocephalus occurs there. The D. pepinensis (Fig. 128, a) 

 is a typical species. There were many smaller trilobites, among which were 

 species of Ptychoparia, Conocoryphe, Crepicephalus, Agraulus, etc. Brachiopods 

 were well represented by phosphate linguloid shells, of which Lingulepis 

 pinniformis (Fig. 128, e and /) and Obolella polita (Fig. 128, g) may be taken as 

 typical, and by shells of calcium carbonate of which Plectorthis newtonensis (Fig. 

 128, b and c) and Billingsella coloradoensis (Fig. 128, m and n) are examples. 

 The Hyolithes (Fig. 128, d) were still common. Gastropods were abundant in favor- 

 able localities, and were represented by shells of the cap type, by those coiled into 

 a spire, as Holopea sweeti (Fig. 128, h), by those coiled in a plane, as Bellerophon 

 antiquatus (Fig. 128, i and j), and by those having a depressed coil, as Ophileta 

 primordialis (Fig. 128, k and I). Pelecypods and cephalopods were rare; as were 

 also corals and cystoids. Annelids are quite frequently represented by borings 

 and occasionally by mounds heaped about the openings of these and by trails 

 leading away from them, as shown in Fig. 122. Greensand {Glauconite, a silicate 

 of potash and iron) is rather abundant in certain horizons of the Upper Cambrian. 

 As much of the glauconite of later beds is associated with rhizopod shells, and 

 is now forming in connection with them, the abundant presence of rhizopods in 

 the Upper Cambrian is inferred. Near the summit of the Cambrian series, as it 

 is usually defined, graptolites (Fig. 124, e) become abundant and wide-spread, 

 and characterize a horizon known as the Dictyonema beds. 



The foreign Cambrian faunas. — There is a rather close correspond- 

 ence between the faunas of the eastern American province and those 

 of the western and southern European provinces, which is but another 

 way of saying that the faunas around the border of the North Atlantic 



