[{marraew] ORDOVICIAN SYSTEM ON THE ATLANTIC COAST 265 
ridge, divided lengthwise by a narrow septum, which extends from the 
middle of the valve half way to the front. 
The imprint of the vascular trunks in this valve begin about one- 
third from the back-and extend well forward on each side toward the 
front margin; there are about five branches turning inward toward the 
visceral cavity and numerous branches turned outward. 
As indicated by Billings’s name the valves of this shell are thick ; they 
consist of four to six layers which are separated by a pulverulent substance ; 
the spaces are perhaps cavities filled by infiltration subsequent to the 
entombment of the shells; the thickening cf the shell is mostly posterior 
to the central muscles, though the margins of the pseudo-platform out- 
side of these muscles are thickened. 
The shell, by its massiveness and the arrangement of the muscle- 
scars, shows affinities with the Trimerellidæ, though we cannot refer it to 
any particular genus of that family. Its general form is that of Dino- 
bolus and of the Dinoboli tigured by Messrs. Hall and Clarke, it appears 
to be nearest to D. Conradi,' but it differs from all these by the oval form 
of the main scars of the central group, which are thus more like the 
central scars of Lingulella, so far as in that genus the dorsal valve is 
concerned. 
The orbicular form, narrow mantle margins and heavy lateral 
muscles, give this shel! a superficial resemblance to the Obolidæ, but the 
arrangement of the muscles, especially those of the ventral valve, is quite 
different. 
Sculpture —There is nothing to be added to Mr. Billin 
of the external features or surface markings of this shell. 
Size.—Length of the dorsal valve 32 mm. Width 33 mm. The 
ventral valve is about 2 mm. longer than the dorsal. 
Locality.—Great Bell Island in Conception Bay, Newfoundland, with 
the preceding. 
ss description 
as’ 
gss 
This species is distinguished from the preceding by the advanced 
position of the muscle-scars and by having much narrower mantle 
margins, as well as by its general form. 
The genus Dinobolus, to which this is allied, is found both in the 
Ordovician and Silurian systems. Dinobolus Conradi belongs to the Nia- 
gara group. 
In endeavouring to determine the geological horizon of the sandstones 
in the islands of Conception Bay one is left in uncertainty by the few 
species found there and the small collections of fossils obtained from those 
islands, The fossils were formerly referred to the Potsdam sandstone, as 
they were thought to have a Primordeal aspect. But Cruziana and 
Eophyton, which occur there, are tracks or trails of animals, and the 


HOPrCle. Pl VAI es 13 FON. 

