[DAWSON] FOSSIL SPONGES AND OTHER ORGANIC REMAINS 



119 



<;;imi(' discs oi' pyrite. or may ropi-esent flattened »)ru;anic bodies. In the 

 liitloi- rase, they may l)e ii;eiMns or ova. or s])()res, or portions of some 

 foniplox \'Oif«'tabie or animal ori^-anism Tlioy recall the little spherules 

 described by Matthew as occurrimr in the Cambrian of Xew Brunswick, 

 and named by him Monddifrs (//i)l)u/(iris. I have not been able, liowever, 

 to detect any indications of coiniectinii- rods or stalks like those figured 

 bv Matthew.' 



Vr. OTHER ANIMAL EEMA1N8. 



The most important of these are the little brachiopods referred to 

 above : 



Obolella (Linnakssonia) pketiosa, Billinifs. 



"b c 



Fi(i. '.i\.—TAiinarssonia prei iosa, Billings, a, natural size of medium specimens, 



6, ventral, c, dorsal valvf. 



These are very alnmdant on tlie 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 Sillerv,and 

 the genus is a Cambi-ian rather than Ordovician one in Europe. For this 

 reason I ([Uote here Dr. Hall's description, based on Metis specimens 

 submitted 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. pretinm : 



■• Shell smail.subcircularorelongate transversely. Valves subeqmdiy 

 convex, the ventral beak ei-ect. slightly projecting and perforated at its 

 apex. Externa) surface covered with fine concentric lines, faint radia- 

 ting .stria^ being visiiile on the interstitial lamelhe. Tiie interior of the 

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

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

 of the branches is penetrated by the foraminal lube. 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." 



Transactions Royal Society, 1889. 



