6 Proceedings of the Royal Physical Society. 



Nautilus; next in order are the Gasteropods, chiefly 

 Maclureas, Pleurotomarias, etc. ; while the lamellibranchs 

 and brachiopods rank last in point of number. The two 

 latter are found with their valves attached, and the lamelli- 

 branchs are found in the position in which they lived and 

 died. All the specimens show that every open space into 

 which the mud could gain access and the worms could crawl 

 are traversed by worm casts. In the case of the orthocera- 

 tites, they seem to have lain long enough uncovered by sedi- 

 ment to allow the septa to be dissolved away from the 

 siphuncles which they held in place. Many of those 

 siphuncles are now found isolated; indeed, Salter founded 

 his genus Piloceras on such large examples as those found in 

 Endoccras. Sponges of the genus Archmocyathus and 

 Ccdathium occur at intervals in the muddy matrix. One 

 example is preserved in chert ; but the larger masses of chert 

 in the limestone do not seem to be derived from sponges, but 

 more probably from the siliceous skeletons of diatoms, which, 

 in all likelihood, were as abundant in that ancient ocean as 

 they are now. No undoubted remains of foraminifera have 

 been discovered, though on several horizons there are zones 

 of limestone made up of small rounded bodies, probably 

 oolites ; but owing to the fact that the limestones are crystal- 

 line, and that many of them have been more or less dolomi- 

 tised, it is now almost impossible to decide definitely as to 

 the nature of these spherules. For these reasons it is hope- 

 less to find minute organisms in this formation. The shell 

 substance of the larger fossils has in almost every case been 

 dissolved out, and the spaces have been filled with calcspar, 

 and in some cases with Beekite, so that all the finer markings 

 on their surfaces are obliterated. 



In view of the foregoing evidence, it is clear that the 

 ph3^sical conditions which prevailed during the deposition of 

 the Silurian strata in the north-west Highlands must have 

 been remarkably uniform. As already indicated, the fossils 

 are of an American type, and do not resemble those found 

 in the contemporaneous deposits of Wales and England. 

 Indeed, so far as the order of succession of the beds and 

 their fossil contents are concerned, we have an almost exact 



