246 On Fossil Rain Drops. 



several concentric series, and also in their microscopical 

 structure, as well as in their mode of attachment to the 

 skin lining the jaw, and not to the bone itself. We have 

 other liints of the relation between Cyclostomes and Pla- 

 giostomes in their spiracles, and also in their numerous 

 respiratory apertures, so that, after due consideration, I 

 come to the conclusion that the Myxinoids and Petromy- 

 zons, far from being the types of peculiar sub-classes, are 

 simply embryonic forms of the great type to which sharks 

 and skates belong, bearing to these powerful animals, in a 

 physiological point of view, the same relation which exists 

 between Ichthyodes and the tailless Batrachians. 



Of Cyclostomata, two species have been mentioned as oc- 

 curring in the colder parts of North America, both referred 

 by Dr Richardson to the genus Petromyzon proper, but of 

 which I have seen no trace myself in the great lake region, 

 though I know Petromyzons to occur below Niagara Falls. 

 However, I am able to add a new species of this family to 

 the fauna of those waters, which belongs to the genus Am- 

 mocsetes, and was found in the mud in Michipicoton River, 

 at the landing-place of the factory, the first specimens of 

 which were picked up by the students, when dragging their 

 canoes along the shore. — Agassiz on Lake Superior^ p. 249. 



On Fossil Bain Drops* 



Mr Desor communicated some observations made by Mr 

 Whitney and himself in reference to the probable origin 

 of the so-called fossil rain drops, which, in this country, we 

 found on slabs of red sandstone, as well as Potsdown sand- 

 stone. 



He said it had already been noticed by Mr Teschemacher 

 that these so-called rain drops, when closely examined, are 

 found to differ in several respects from the impressions made 

 by the rain on a beach, where each drop produces an impres- 



* Proc. Boston Soc. Nat. Hist., 1850. 



