Address by Professor B. N. Peach. 459 



Above this horizon, at a height of over 800 feet above the top of the 

 OlettelluH zone, we encounter the great home of the fossils peculiar to 

 the Durness Limestone in the Balnakeil and Croisaphuill groups. The 

 former consists mostly of dark limestones, with nodules of chert, and, 

 with a few alternations, of white limestone bands. A few thin layers 

 are charged with worm casts. The overlying group is more varied, the 

 lower part being composed of dark-grey limestones full of worm casts, 

 and with some small chert nodules arranged in lines ; the middle 

 portion, of dark granular and unfossiliferous dolomite ; and the upper 

 part, of massive sheets of fossiliferous limestone full of worm casts. 

 The total thickness of these two groups in Durness is about 550 feet. 



These two subdivisions have yielded over twenty genera and about 

 one hundred species. In Durness sixty-six species have been obtained 

 from the Balnakeil group alone, fifteen of which have not as yet been 

 found in the overlying Croisaphuill group, thus leaving fifty-one species 

 common to both divisions. The Ben Suardal limestones in Skye, which 

 were mapped by the Geological Survey as one division, are regarded, 

 on palseontological grounds, as the equivalents of both these groups. 

 Owing to the number of species common to both subdivisions, the fauna 

 will be here referred to as a whole. 



Both siliceous and calcareous organisms are present in this fauna. 

 Aniong the former we find Archceoscijfhia (Hinde), described by Billings 

 as ArchcBOcyathus, an early Cambrian coral, but shown by Hinde to be 

 a siliceous sponge. ^ The genus Calaihium is represented by four species. 

 Other genera and species of sponges occur, so that the siliceous nodules, 

 which are very common in both groups, may be in great part due to 

 them. In this connexion it may be mentioned that Hinde obtained 

 sponge spicules from some of the nodules. Hinged Brachiopods have also 

 been collected from these beds, and include Nisusia (^Orthosina) festinata, 

 N. grandceva, and Camarella. 



But the characteristic feature of the fauna is the assemblage of 

 calcareous Mollusca comprising Lamellibranchs, Gasteropods, and Cephalo- 

 pods, showing a wide range of variation, and consequently a long 

 ancestry. The Lamellibranchs, though represented only by two genera, 

 Euchasma and Eopteria of Billings, with several intermediate forms, 

 are of extreme interest, as they are only known to occur elsewhere in 

 Newfoundland and Eastern Canada. The Gasteropods, however, furnish 

 the largest number of species — about 48 per cent of the whole. The 

 primitive Euomphalids, Maelurea and Ophileta, are most characteristic. 

 The former genus has a large number of species, many of which are 

 to be found in the Beekmantown Limestone of Newfoundland and Eastern 

 North America. Only one of the species (^Maelurea Peachi) is peouliar 

 to Durness. Several species of Ophileta are found, some of which like- 

 wise occur in the Beekmantown Limestone. Euoinphalus has also been 

 recorded, while several forms belonging to the nearly allied family of 

 the Turbinidaj, and placed in Lindstrom's genus Oriostoma, are also 

 met with in the Beekmantown Limestone. 



Murchisonids and Pleurotomarids number twenty-seven species and 

 show a very wide range of variation. The chief sub-genera of the 

 former are H-ormotoma and Ectomaria, many species of which occur 

 with remarkable variations. All the types of variation found in Durness 

 are to be found in North America, and several of the species are 

 common to both regions. The Pleurotomarids vary in a similar manner, 

 the chief genera being RapJiistoma and Euconia, and a form resembling 

 B-ormotoma, only with a shorter spire. Species belonging to each of 

 these sub-genera are likewise common to both areas, while some are 

 only known from the North- West Highlands. 



The Cephalopods are of equal interest. They are also of primitive 

 type, and, at the same time, show a wide range in form. The 



' Quart. Journ. Geol. Soc, vol. xlv, p. 125, 1889. 



