418 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 1, 



EXPLANATION OF PLATE XIII. 



Fossils from the Limestone of Durness. 



Fig. 1 a. Maclurea Feachii, Salter. Under or flat face, with the operculum 

 added (1 i) to show its normal position. 



Fig. 2. Upper or convex side of another specimen. 



Fig. 3. Eestored outline, seen from the front, with the operculum in 



position. 



Fig. 4. Operculum, upper view. 



Fig. 5. Operculum, viewed from within. 



Fig. 6. Operculum of M. Logani, Salter. For comparison. In these figures, 

 a is the straight lower edge ; b, the umbo ; c, the inner lower angle ; 

 d, the upper angle. 



Figs. 7, 8. Murchisonia gracilis, Hall ; var. gracillima. 



Figs. 9, 10. M. angulocincta. 



Fig. 11. M.bellicincta, Hall? 



Fig. 12. Ophileta compact a, Salter, a, the lower face ; c, the upper concave 

 face ; h, the convex dome-like cast of the upper surface. 



Fig. 13 a. Fleurotomaria Thule, Salter. A portion of the surface, 136, magnified 

 twice. 



Fig. 14. Orthis striatula, Emmons, a, dorsal valve ; h, a portion of the same 

 magnified. 



Fig. 15. Dorsal valve. 



Fig, 16. Ventral or convex valve. 



Figs. 17-21. Filoceras invaginatum, Salter. Fig. 17 is a specimen of a nearly 

 perfect mouth, drawn behind the abraded specimen, fig. 18, which 

 shows the concentric septa on the rough section, and is obscurely 

 annulated on the outside ; fig. 19 is the conical curved tip ; fig. 20 

 shows a section of the cap-like septa in a fractured specimen ; fig. 21 

 gives a transverse view of another specimen, showing three concentric 

 septa. 



Fig. 22. Orthoceras vertehrale, Hall ? or young of 0. bilineatum, Hall ? Com- 

 pressed and bent specimen. 



Fig. 23. 0. t;erzfeirafe, Hall? 



Fig. 24 a. Orthoceras m^ndax, Salter. Broken, and showing the subcompressed 

 siphuncular tube; 245, transverse view of concave septum with 

 siphuncle, from near the smaller end. 



Fig. 25. Orthoceras undulostriatum, Hall. Interior view with angularly bent 

 septum, 



Fig^ 26. Front (or ventral ?) view of another specimen. 



Fig. 27. Oncoceras ? undescribed species. A very doubtful specimen. 



Fossils from the Qwartz-rocTc and Shale. 



Fig. 28. Annelide-tube {Helminthites), or trail. 



Fig. 29. Double burrows {Arenicolites) in quartz-rock. 



Fig. 30. Side view of another specimen, showing the contents of the bur- 

 rows, roughly striated in a vertical direction. 



Fig. 31. Serptclites? M'Cullochii,M.wi'chison. In a mass of sandstone. These 

 thick, short, free annelide-tubes are very common in the quartzose 

 sandstones of Durness. 



