460 Notices of Memoirs — Dundee, 1912 — 



prominent feature in the straighter specimens is the great width of 

 the laterally placed siphunclc, which is generally furnished with 

 endocones and organic deposits. The genus Piloceras is the most 

 characteristic type, and shows this peculiar feature best. It has only 

 been recorded from Scotland, Newfoundland, Canada, and the eastern 

 States of North America. The following additional genera are repre- 

 sented, viz., ^«fZc>ccr«.'!, chiefly by siphuncles in great variety, Actinocerag. 

 Cyrtoceyas, and, doubtfully, Orthoceras. Several forms have been 

 attributed to Orthoceras, which, on re-examination, have been found to 

 be the siphuncles of other genera, resembling American types described 

 by Hall and Whitfield. 



The whorled Nautiloids provisionally classed with the genus TrocJwlites 

 of Conrad are represented by several distinct forms as yet unnamed. 



The Trilobites are of rare occurrence in these two groups of dolomite 

 and limestone. They are fragmentary and poorly preserved. This 

 is doubtless one of the disappointing features connected with this 

 remarkable assemblage of organic remains, for the presence of a zonal 

 form would have helped to define the horizon of the=e beds. Only one 

 species, Bathyurus Nero (Billings), has been identified, whioh also occurs 

 in the Beekmantown Limestone of Newfoundland. The other Trilobite 

 remains, though poorly preserved, have a Cambrian fades characteristic 

 of North America. 



In connexion with this fauna certain features have been observed 

 which throw some light on the absence of calcareous organisms from 

 thick zones of the Durness dolomite and limestone. In my detailed 

 description of the palaeontology of the Cambrian rocks of the North- 

 West Highlands in the Geological Survey Memoir, I stated that " in 

 most cases the septa and walls of chambered shells have been wholly or 

 in part dissolved away, so as to leave only the more massive structures 

 of the siphuncles, and worm castings are often found within the chambers 

 where the septa have been preserved. These features seem to indicate 

 that the accumulation of the calcareous mud in which the fossils were 

 embedded was so slow that there was time for the solution of part 

 of an organism before the whole of it was covered up ".^ There is 

 good reason to believe that many organisms wholly disappeared by this 

 process, so that it is reasonable to conclude that the fossils obtained 

 from the Durness dolomites cannot be regarded as furnishing a complete 

 life-history of the forms that originally existed in that sequence of 

 deposits. Attention has already been called to the fact that beneath 

 the two subdivisions now under consideration there are groups of 

 dolomite and limestone which so far have yielded no organic remains 

 beyond worm castings. And even in the important Croisaphuill group, 

 with its fossiliferous zones, there are thick groups of dolomite which 

 have furnished no calcareous organic remains. Obviously the palajonto- 

 logical record in this instance is glaringly incomplete, for we have no 

 reason to suppose that the life of the time flourished in some of the 

 calcareous zones and not in others. 



The highest subdivision of the Durness Limestone, measuring about 

 150 feet in thickness (Durine group), has yielded two species of 

 Ilormotomn — viz. IL. gracilis and //. gracillima — both of which occur 

 in the two underlying groups. //. gracilis occurs in the Beekmantown, 

 the Chazy, and the Trenton -Limestones of America. 



Before assigning any stratigraphical horizons to the fauna of the 

 Durness dolomites, it is desirable, owing to the American facies of the 

 fossils, to recapitulate the evidence bearing upon the life of Cambrian 

 time in North America. But the Cambrian life-history of Scotland 

 would be incomplete without a brief reference to the recent discovery 

 of fossils along the eastern border of the Highlands. 



^ Geological Structure of the North-West Highlands : Geol. Surv. Mem., 

 1907, p. 380. 



