JURASSIC PERIOD. 433 



Rhsetic beds, of the Upper Triassic, which directly underlie the lowest 

 Jurassic. Like Ammonites, they are exceedingly common fossils in 

 the Mesozoic age, and are unknown afterward. 



An Ammonite of the American Jurassic is represented in Figs. 759, 

 759 a, the latter a side view. One of the five-sided disks of the stem 

 of a Pentacrinus, a genus of Crinoids characteristic of the Mesozoic, 

 is shown in Fig. 755. The triangular shell, represented in Fig. 757, 

 belongs to the genus Trigonia, which is characteristic of the early 

 and middle Mesozoic. Here also begin the genus of the Oyster 

 family, called Gryphcea, in which the beaks are incurved, and the 

 large pearly Inoceranitcs, both peculiarly Mesozoic types. 



Characteristic Species. 



Plants. — No plants have been described, "except a few by Newberry, from a coal 

 seam in the gypsiferous sandstone of the Upper Colorado, in the Moqui country (near 

 the meridian of 111 ), the age of which is doubtful. The observed genera are Cyclop- 

 teris, Pecopteris, Neuropteris, Sphenopteris, and Clathropteris. 



The Clathropteris from near the middle of the Connecticut River sandstone (Fig. 707, 

 p. 407), as suggested by Hitchcock, is some evidence — though far from decisive — for 

 referring the upper half of that formation to the Jurassic. The European species of 

 this genus occur in the Lias and Trias. 



Animals. — 1. Radiates. — Fig. 755, a joint of the stem of Pentacrinus asteris- 

 cus M. & H., a Crinoid with a pentagonal column. 



2. Mollusks. — (a.) Lamellibranchs. — Fig. 756, Monotis curta, from the Black 

 Hills; Fig. 757, Trigonia Conradi M. & H., ibid.; 758, Tancredia Warreniana M. & 

 H., ibid, (b.) Cephalopods. — Fig. 759, young specimen of Ammonites cordiformis M. 

 & H., ibid.; Fig. 759 a, side-view of the same; Fig. 760, Belemnites densus M. & H., 

 the upper part broken away, ibid. 



The Jurassic beds of Genesee valley, Plumas County, California, contain a Belemnite, 

 Trigonia pandicost a M., a Gryphma near G. vesicularisJir., Tnoceramus (?) obliquus M., 

 /. (?) rectangulus M., Rhynchonella gnathopliora M-, and others of the genera Lima, 

 Pecten, Mytilus, Astarte, Unicardium, Myacites, and Terebratula. In the beds of the 

 Uintah Mountains, Marsh found specimens of Pentacrinus asteriscus, Belemnites densus, 

 a Trigonia, and other Mollusks, and also the right humerus of a small Crocodilian. 



Among the Arctic fossils of this period, there are, at Prince Patrick's Land, Am- 

 monites M' Clintocki, a species near A. concavus Sow., of the Lower Oolite; and at 

 Cook's Inlet, Ammonites WosnessensM, A. biplex Sow. (?), Belemnites paxillosus (B. 

 niger~List ?), and Pleuromya unioides Br. (Unio liassinus Schubler). A. biplex also 

 is reported to occur in the Chilian Andes, in latitude 34° S., and probably also in Peru 

 near the equator, as well as in Britain and Europe. 



2. Foreign. 

 I. Rocks : kinds and distribution. 



The strata of the Jurassic period in England (see map, page 344, on 

 which the areas numbered 7, 8 are Jurassic) appear at the surface 

 over a narrow range of country (averaging thirty miles in width), com- 

 mencing at Lyme-Regis and Portland on the British Channel, and 

 extending across England, north of northeast, to the river Humber, 

 and still farther north, on the eastern coast of Yorkshire, almost to 



28 



