Gestation in Batrachians and Fishes, 49 



bable explanation of the only fact which might be considered at 

 variance with the conclusion stated above, viz., that we have in 

 these fishes a mouth gestation. In the ma?s of eggs with which 

 the mouth is filled, I have occasionally found the eggs, rarely 

 more than one or two, of another species. The only way in 

 which their presence may be accounted for, it seems to me, is by 

 the supposition that while feeding, the eggs are disgorged, and as 

 these fishes are gregarious in their habits, when the ova are re- 

 covered, the stray egg of another species may be introduced into 

 the mouth among those which naturally belong there. 



ARTICLE VI. — Description of some new species of Fossils from 

 the Lower and Middle Silurian Rocks of Canada. By E. 

 Billino^s. 



(From the Report of the Geological Survey for 1860.) 



In the Silurian Rocks of Canada and the neighbouring coun- 

 tries there are maiiy species or varieties of that group of the 

 genus Strophomena of which S. alternata may be regarded 

 as the typical form. These are all closely related and yet exhi- 

 bit such differences that only those naturalists who entertain 

 wide views upon the subject of the value and significance of 

 specific distinctions, would feel inclined to unite them under one 

 common name. The forms of this group most common in the 

 Lower and Middle Silurian Rocks are S. alternata^ S, incrassata, 

 S, deltoidea^ S. camerata, S. tenuistriata and some others to be 

 described hereafter in this paper. The first of these ranges 

 from the Chazy limestone upwards perhaps to the Niagara rocks 

 but is most abundant in the Trenton limestone and Hudson River 

 group. It is also very widely distributed, as it occurs in all 

 parts of the Continent, where the last two formations have been 

 recognized and is also found in the Lower Silurian in England 

 and Ireland. S. incrassata has exactly the same form as some of 

 the varieties of aS'. alternata but is never, as far as I have been 

 able to ascertain, more than half the average size of this latter 

 species. It seems to be confined to the Chazy and the Black River 

 limestone or the lower part of the Trenton, and has therefore, a 

 geological distribution diff'erent from that of S. alternata^ a fact 

 which would appear to constitute an additional ground for classi- 

 fying it as a distinct species. S. deltoidea is a Trenton lime- 



Canadian Nat. 4 Vol. V. No. y 



