252 



whether before or after the coal, by examination of the floras, 

 and finding the Permian resembling the carboniferous, he would 

 place the limit between the carboniferous and the Devonian at 

 the old red sandstone ; with the tertiary came in the reign of 

 mammals, the marsupials having come in during the oolite. 



In the number of species which he has used to characterize a 

 distinct fauna, he has always taken into account the previously 

 ascertained abundance of any fossils under consideration in other 

 geological periods, and whether the family were in the ascen- 

 dency or on the decrease at the period examined ; without the 

 elimination of errors which might arise from these sources, any 

 observations would be of little value. In his mode of determin- 

 ing or identifying species, he always ascertains, if possible, 

 whether they belong to a genus in which the species vary little 

 or much, (as in the herrings in the former case, and the percoids 

 in the latter,) and whether to a genus in which there are or are 

 not great differences in size. 



In regard to the vitality of eggs, the President remarked that 

 it is well known that the eggs of salmon and trout are sent by 

 diligence all over France ; and Prof. Rogers observed that the 

 flight of most birds would enable them to multiply on both sides 

 of a narrow strait like that alluded to. 



Mr. Theodore Lyman presented the following descrip- 

 tions of new Ophiuridce, belonging to the Smithsonian 

 Institution and to the Museum of Comparative Zoology 

 at Cambridge. 



Ophiothrix, (Mull. & Trosch.) 



Opliiothrix dumosa, (Lyman,) sp. nov. 



Description of a Specime^i. Diameter of disc, 11.5 millim. 

 Length of arm, 72 millim. Teeth 6, cutting edge a little 

 rounded ; squarish, thick ; sometimes split in two ; upper one 

 rather narrower and more tapering. Tooth-papillte about 38, 

 arranged in two diverging, principal rows, between which are 

 included some smaller ones ; these two rows are vertical, and 

 start, just below the teeth, with a pair of stout papillae, like a 

 tooth split in two. Mouth-shields oval heart-shaped, with a de- 



