ON FOSSIL SPONGES FEOM METIS. 65 



also groups of more miuute rounded bodies with uo distiuct structure, except iu a few 

 cases an apparent notcli at the margin. They may be spore-cases or ova, but perhaps 

 are not organic. 



An interesting point in connection with these remains is the appearance of so many 

 distinct types of siliceous sponges in one locality and formation, and this of so great age. 

 It is also deserving of note that these sponges are of types usually occurring in deep 

 water, and if we regard the dark shales containing them as deep-water deposits, this 

 might account for the absence of other fossils. The alternation of these shales with 

 coarse conglomerates and sandstones woiild also imply great oscillations of level at the 

 time of their deposition. 



The occurrence of so many species of sponges in very thin layers of shale, for the 

 most part unfossiliferous, iu connection with the obscure and unobtrusive character of 

 these remains, is also an indication of the importance of thorough study of the older 

 formations, and of investigation of even their more unpromising portions, as well as of 

 the exhaustive exploration of those portions in which fossils occur. 



NOTE. 



Description of Lmnarssonia cnf. pretiosa, Billings. 



By Prof. James Hall, LL.D. 



Shell small, subcircular or elongate transversely. Valves subequally convex, the ventral beak 

 erect, slightly projecting and perforated at its apex. External surface covered with fine concentric 

 lines, faint radiating strias being visible on the interstitial lamellte. The interior of the ventral valve 

 bears a snbtriangular or U-shaped ridge, the branches of which diverge anteriorly. The thickest 

 portion of this ridge at the union of the branches is penetrated by the foramina! tube. In front of 

 the foramen, and just within the cardinal line, on either side the axis of the shell is a conspicuous 

 tubercle or boss. In the dorsal valve is a median ridge, extending half the length of the valve, and 

 from this two short lateral ridges diverge, taking their origin at one-third the length of the median 

 ridge from the posterior margin. 



(The above description has been kiudly supplied by Prof. Hall from specimens sent to him from the Peter 

 Redpath Museum.— J. W. D.) 



