432 



MESOZOIC TIME. 



all 360 feet in thickness. In the Wahsatch, Jurassic beds occur on the eastern side, 

 beneath the Cretaceous. 



In the auriferous slates of the Sierra Nevada, on the Mariposa estate, there occur 

 Aucella Errinytoni Gabb, Pholadomya (?) orbiculata Gabb, Belemnites Pacificus Gabb; 

 and other species in Genesee Valley, and probably at Spanish Flat, El Dorado County. 



The Arctic localities are — the eastern shores of Prince Patrick's Land, in 76° 20' N., 

 117° 20' W.; the islands Exmouth and Talbe, north of Grinnell Land, 77° 10' N., 95°' 

 W.; and Katmai Bay, or Cook's Inlet, in Northwest America, 60° N., 151° W. 



II. Life. 



Several of the genera of Radiates and Mollusks which mark the Ju- 

 rassic beds of Europe have been found in the American Jurassic, the 

 most prominent of which are Pentacrinus, Trigonia, Ammonites, and Be- 



Figs. 755-760. 



Fig. 755, a segment of the column of Pentacrinus asteriscus ; 756, Monotis curta; 757, Trigonia 

 Conradi ; 758, Tancredia Warreniana ; 759, Ammonites cordiformis ; 759a, Side-view of same, a 

 little reduced ; 760, Belemnites densus. 



lemnites. The characteristics of Ammonites are briefly mentioned on p. 

 124, and again on p. 409. The fossil Belemnite (Fig. 760) is the in- 

 ternal bone, or osselet, of a Cephalopod, answering to the pen of the 

 squid (Fig. 159, p. 119). They are much heavier than the same part 

 in any modern species. The fossil represented in Fig. 760 is really 

 only the lower and stouter part of the osselet : its structure is radiately 

 fibrous. It has a conical cavity (or alveolus) within, opening upward, 

 and at the bottom of this cavity there is, when it is perfect, a small 

 chambered cone called the phragmocone, which has a siphuncle. The 

 osselet, when unbroken, has a thin edge, and is further prolonged on 

 one side into a delicate concave blade, a variety of which is shown in 

 Fig. 792, on p. 440. The animal was much like that of a Sepia (see 

 Fig. 159, p. 119) ; and its ink-bag was contained within the cavity of 

 the osselet. The first species of the genus known was found in the 



