[DAWSON] FOSSIL SPONGES AND OTHER ORGANIC REMAINS 



119 



i^unic discs ol' pyrite, or may repreMent flattened organic bodies. In tlio 

 latter case, they may be ujerms or ova, or spores, or portions of some 

 complex vegetable or animal orijanism They recall the little spherules 

 described by Matthew as occui'riii<^ in the Caml»rian of New Brunswick, 

 and named by him Monaditca (jlolmUirls. 1 have not been able, however, 

 to detect any indications of connectinj^ rods or stalks like those fi<^ured 

 by Matthew.' 



VI. OTHEE ANIMAL EEMAINS. 



The most important of these are the little brachioi)od8 referred to 

 above : 



Obolella (Linnarssonia) pretiosa, Billings. 

 h e 



Fig, 31.— Linnursaonia pretiosa, Billings, a, natural size of medium specimeis, 



6, ventral, c, dorsal valve. 



These are very abundant on the same surfaces which hold the sponges, 

 and are usually replaced by pyrite. though sometimes appearing as mere 

 casts. They often show their interior structures as represented in Fig. 

 31. They afford for the present the best evidence in relation to the geo- 

 logical age of the deposit, since the species is characteristic of the Siliery , and 

 the genus is a Cambrian rather than Or(U)vician one in Europe. For this 

 reason I quote here Dr. Hall's description, baseci on Metis specimens 

 siibmitted to him some years ago. I may add that Mr. Whiteaves, our 

 best Canadian authority, concurs in referring these shells to Billings's 

 species 0. j)retiosa : 



" Shell small, subcircular or elongate transvereely. Valves subequally 

 convex, the ventral beak erect, slightly projecting and perforated at its 

 apex. External surface covered with tine concentric linos, faint radia- 

 ting stra? being visible on the interstitial lamelhr. The interior of the 

 ventral valve bears a subtriangular or U'^^^^^'P^d ridge, the branches of 

 which diverge anteriorly. The thickest portion of this ridge at the union 

 of the branches is penetrateil by the foraminal 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 doreal 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." 



1 Transactions Royal Society, 1889. 



