TRENTON PERIOD. 209 



Brachiopods in the profusion of their relics; (4) Trilobites (figs. 320 

 -326), which are greatly multiplied in genera and numbers of 

 species, and attain in some cases a gigantic size; (5) Bryozoans, a 

 group including a multitude of delicate corals having minute cells 

 (figs. 266, 267). 



There were but few Polyp-corals, compared with the number in 

 later periods. Single masses of the coral Coiumnaria alveolate H. 

 (fig. 278) occur in the Black Eiver limestone, weighing between 

 two and three thousand pounds. Delicate, plume-like fossils, called 

 Graptolites (p. 190 and fig. 281), were a feature of the seas, though 

 still more common in the next, or Hudson, period. Cystids (figs. 

 265, 285) were the most characteristic kind of Crinoids. They 

 belong in geological history eminently to this early era, reaching in 

 it their greatest expansion. The Brachiopods were small species 

 in the first epoch (figs. 268-270), but large and of many kinds in the 

 following (figs. 286-300). The Trenton species were mostly of the 

 Orthis family (the genera Orthis, Orthisina, Leptcena, and Strop homena) ; 

 and with these there were species of the Lingula, Discina, and Rhyn- 

 chonella groups, — the same families that were represented in the 

 earlier Calciferous epoch. The genera Rhynchonetta (fig. 270), which 

 begins in the Chazy, and Crania (fig. 330), of the Trenton, have 

 representatives in modern seas. 



The small bivalve Crustaceans Leperditice, (fig. 276) occur in im- 

 mense numbers in some places, as on the Ottawa in Canada, where 

 a bed of limestone two feet thick is wholly made up of them ; and 

 yet the length of the shell is only one-ninth of an inch. 



The huge Orthocerata exceeded in magnitude any existing Cepha- 

 lopods, and were the great animals of the Trenton world, — the first 

 in size and rank. 



Characteristic Species. — 1. Chazy Epoch. 



1. Protozoans. — Sponges. — Eospongia Rosmeri and E. varians B. occur at 

 the Mingan Islands. 



2. Radiates. — (a.) Polyps. — No species have been described. (b.) Aca- 

 lephs. — Species of Chsetetes and Coiumnaria (Billings), (c.) Echinoderms. — The 

 Crinoids include as many known Cystids as Crinids. The following are a few 

 of them : — 



(1.) Crinids. — Palseocrinus striatus Billings (fig. 264), the body, showing the 

 radiating ambulacral grooves (five) at top ; Blastoidocrinus carcharidens B., — 

 the genus apparently of the Pentremite family, a family which makes its next 

 appearance in the middle Devonian and abounds in the Subcarboniferous. — 

 (2.) Cystids: — Malocystis Murchisoni B. (fig. 265), the body nearly spherical 

 (whence the name, from the Latin malum, an apple), and having no arms, and 

 the ambulacral grooves irregularly radiating. Another Chazy genus is tha 

 Palseocystis B., which includes Hall's Actinocrimis tenuiradiatus. 



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