208 PALEOZOIC TIME. 



Ctenodonta varicosa. Orthonota nasuta Con., Conocardium dipterum S. (Fig. 391), 

 Ambonijchia Triton. — (4) Pterqpods and Beterqpods: Theca triangularis Portl., T. 

 vaginula S., Ecculiomphalus Bucklandi Portl., Bellerophon carinatus Sow. — (5) Gaster- 

 opoda: Maclurea Logani S.? (Scotland), Murchisonia simplex M'Coy, ffolopea con- 

 c'mna M'Coy. — (G) Cephalopoda: Orthoeeras vagansS.; Lituites Hibernicus S. ; Cyrto- 

 ceras iruequiseptum Portl., C. multicameratum H. ? 



Articulates. — (1) Worms: Nereites Sedgwickii Murch., Tentaculites Anylicus S. 

 (2) Trilobites: Ogygii Buchii Brngt., Asaphus tyrannus Angelin, A. Poicisii Sharpe 

 (Fig. 392), Trinucleus concextricus, Calymene Blumenbachii, Illamus Davisii 

 S. (Fig. 393), Ampyx nudus Murch. (Fig. 394), Lichas Hibernicus Portl., Agnostus 

 pisiformis (also Primordial), and also species of Harpes, Phacops, Cheirurus, Cybele, 

 etc. — (3) Ostracoids : Beyrichia complicata. 



The Lower Llandovery rocks contain Stricklandinia (Pentamerus) lens*, and 

 rarely Pentamerus oblongus*, both species occurring in the Anticosti beds, also 

 Pentamerus undatus Sow., Meristella angustif'rons M'Coy, Ml 1 crassa Sow., Atrypa 

 reticularis*, A. crassa, Orthis calUgramma*f, 0. ELEGANTULA*t, 0. virgata, Stro- 



PHOMENA DEPRESSA*f, LEPTJENA SERICEA*f. L. TRANSVERSALlS*f, Murchisonia 



simplex, Bellerophon dilatatus Sow.*f, Petraia subduplicata*^ ; Illamus Boirmanni S.j 

 Calyjiene Blumenbachii*!, Lichas laxatus M'Coyt, Homalonotus sulcatwf. Fa- 



VOSITES GOTHLANDICA*!, HELIOLITES INTERSTINCTA*f , HALYSITES CATENULA- 



ta*|. The species whose names are marked with a f occur also in the formations 

 below ; and those with an * are found also in the Upper Silurian. A species of Eozoon 

 has been reported from the green serpentine marble of Connemara, of the age of the 

 Lower Silurian according to Murchison, it underlying the Lower Llandovery beds. 



Rhizopods have been found by Ehrenberg in the Obolus or Ungulite grit of Russia. 

 The rock is in part a very soft green-sand ; and the connection of the microscopic 

 Rhizopod shells with the green grains shows, as Ehrenberg states, that it is of the same 

 nature with the Green-sand of the Cretaceous. Among these fossils, occur the three 

 modern genera Textularia, Botalia, and Guttulina. Ehrenberg has also detected in this 

 rock great numbers of Pteropods (related to Hyolites), and made out ten new species 

 and four genera. The rock derives its name from its most common fossil, Obolus Apol- 

 linis (Fig. 246, p. 173 ), which is about as large as a small finger-nail. The Siphono- 

 treta unguioxlata (Fig. 245) is another of its fossils. It has also afforded minute teeth, 

 not larger than pins' heads, which Pander regarded as those of Ganoids, but which 

 have since been shown to be from the dental apparatus of Mollusks. The age of the 

 beds is either that of the Trenton or earlier. They underlie a dark -colored schist con- 

 taining graptolites . and over this occurs the Orthoceratite limestone or Pleta. 



General Observations. 



North American Geography. — The era of limestone-making, — 

 and, therefore, of continental seas largely free from sediments, — which 

 made progress in the Canadian period, reached its culmination in the 

 earlier division of the Trenton period, when limestones were almost 

 the only kind of rock being deposited over the breadth of the conti- 

 nent. The absence of sediments from a large part of the continental 

 region must have been owing to the absence of the conditions on 

 which their distribution depends. The currents of the ocean which 

 ordinarily swept over the land (the Labrador current from the north, 

 along the eastern border, and the Gulf Stream from the south, over 

 the interior), must have had their action partly suspended. This may 

 have been caused by a barrier outside of the limestone area, near or 

 outside of the present Atlantic coast line. If the land, in the shallow 



