TBANSACTIONS OF SECTION C. 



72 



Tlu'v socm to have been developed in sliallow lagoons, and the eh arijct eristic species 

 (if o'liiM)!' these areas rarely appear in another. At the same time the physical 

 ooiiili'ii'" or other causes have o])erated to develop a remarkable variety in I'nrni, 

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

 nccfssarv to arrange them in distinct groups or sections. 



These sections have been made wit li reference to tlie most prominent charac- 

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

 I'lnispicuously marked by a posterior spinifoini extension of the hinge-line, and 

 t'linii tlie section Spirifera. In another group, where the spiniform extension is not 

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

 till' fi'<'ti<in I'lnhnnata. In other forms, the anterior extremity beconu's uasute or 

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

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

 l;iidv oi' I lie sliell, and these ibrms are placed in the section I'atu/afa. Other forms 

 Lave the liinge-line shorter tlian tiiewidtli of tlie shell, the antei'ior end rounded or 

 inincatr, and having the general i'orm of .trcd. Of these, six species are placed 

 iimlir tiie section Amu'deti. Still, a few otlier forms have tlie aspect of Mytilus, 

 and two species are placed under the section Jli/tiiitidcu, Here we have a group 

 (if sliflls among wliicdi we hud no means of generic separation ; and yet within its 

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

 rej.'ar,is form and other external clmracters. The studies of this genus have been 

 made IVom collections of many hundreds of specin:iens 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, 



Tlie f)rms of this genus, as at pri'scnt limited, certaiidy exhibit evidences of a 

 •'.lost remarkable development in form and external characters ; and are well 

 \vortliy the study of the specialist in this department of Pakcontology. 



3. On the Arclimni Uoclx of Girat Britain. Bij Professor T. G. Bonnet, 

 B.Sc, LL.D., F.ii.S., Pres. (?.*S'.— See 'Reports, p. 529. 



4. The Eoznic, lioc/.s of North America.^ 

 Bij T. Sti:iiry Hunt, LL.I)., F.Jl.S. 



Among the pre-( 'ambrian strata of Xortli America there is an invariable 

 succession of crystalline str.'ititied rocks, the constituents of which become pro- 

 irressively less massive and less crystalliiu', 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 

 fuiKlaniental gneiss, ])resent evidences, direct or indirect, of the existence of organic 

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

 jjeneviil title of I'^ozoic, proposed by Sir William Dawson. 'Die term Arcluean 

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

 the nomenclature generally adopted for the great divisions succeeding. 



Tlie following are the subdivisions adopted by the author: — 



Tacanian , , . Quart zite, limestones, and micaceous and argillaceous schists 



(including the Ani)niki('. series of Lake Superior) 4,000 to 



10,000 feet thick. 

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



coarse-grained mica-schists, over 1*0,000 feet thick. 

 llnrnuian . . (-Pebidian) schistose rocks, with stratified 'Gabbros;' 



18,000 feet. 

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



thousand feet thick. 



' I'rinted in full in O'cdl. Mag. die III. vol. i. pj). ."JOG-ijlO, 



