TRANSACTIONS OF SECTION C. C±k 



They seem to have been developed in shallow lagoons, and the characteristic species 

 of one of these areas rarely appear in another. At the same time the physical 

 condition or other causes have operated to develop a remarkable variety in form, 

 and as it does not seem possible to separate these forms generically, it becomes 

 necessary to arrange them in distinct groups or sections. 



These sections have been made with reference to the most prominent charac- 

 teristic of the forms. Of those already known and described, nineteen species are 

 conspicuously marked by a posterior spiniform extension of the hinge-line, and 

 form the section Spirifera. In another group, where the spiniform extension is not 

 conspicuous, the umbo is remarkably prominent, and ten species are arranged under 

 the section Umbonata. In other forms, the anterior extremity becomes nasute or 

 rostrate, and seventeen species are placed under the section Host rata. Other forms 

 are nearly flat and spreading, with little or no extension of the hinge beyond the 

 body of the shell, and these forms are placed in the section Patulata. Other forms 

 have the hinge-line shorter than the width of the shell, the anterior end rounded or 

 truncate, and having the general form of Area. Of these, six species are placed 

 under the section Arcoidea. Still, a few other forms have the aspect of Mytilus, 

 and two species are placed under the section Mytfloidea. Here we have a group 

 of shells among which we rind no means of generic separation ; and yet within its 

 limits the species are developed in the direction of several other genera, so far as 

 regards form and other external characters. The studies of this genus have been 

 made from collections of many hundreds of specimens with the result stated. An 

 examination of a more limited number of examples could easily have led to the 

 reference of some forms to the genus Mytilus, some to Area, and others to new and 

 distinct genera. 



The forms of this genus, as at present limited, certainly exhibit evidences of a 

 most remarkable development in form and external characters ; and are well 

 worthy the study of the specialist in this department of Palaeontology. 



On the Archcean Rods of Great Britain. By Professor T. G. Bonnet, 

 D.Sc, LL.D., F.R.S., Pres.G.S.—See Reports, p. 529. 



4. The Eoznic Rocks of North America.' 1 

 Bij T. Steert Hunt, LL.D., F.R.S. 



Among the pre-Cambrian strata of North America there is an invariable 

 succession of crystalline stratified rocks, the constituents of which become pro- 

 gressively less massive and less crystalline, until we reach the sediments of 

 palaeozoic time ; of which the Cambrian is regarded as the basal member. Since 

 all of these pre-Cambrian rocks, with the exception, perhaps, of the lowest or 

 fundamental gneiss, present evidences, direct or indirect, of the existence of organic 

 life at the time of their deposition, it seems proper to include them under the 

 general title of Eozoic, proposed by Sir William Dawson. The term Archaean 

 appears too indefinite in its signification, and moreover is not in accordance with 

 the nomenclature generally adopted for the great divisions succeeding. 



The following are the subdivisions adopted by the author : — 



Taconian . . . Quartzite, limestones, and micaceous and argillaceous schists 



(including the Anbmkip, series of Lake Superior) 4,000 to 



10,000 feet thick. 

 yiontalban . . Fine grained white gneisses, passing into granulites, or into 



coarse-grained mica-schists, over 20,000 feet thick. 

 Iluronian . . ( = Pebidian) schistose rocks, with stratified ' Gabbros ; ' 



18,000 feet. 

 Arvonian . . . Halleflintas, passing into quartziferous porphyry ; many 



thousand feet thick. 



1 Printed in full in Gcol. Mag. dlt III. vol. i. pp. 506-510. 



