Mr, Hall on Microscopic Shells found in Marl Slate. 295 



This shell bears a close resemblance to a species of Nucula in 

 the Trenton limestone. 



Nucula fahula. Shell twice as wide as long, smooth ; beaks 

 moderately prominent ; crenulations along the hinge-line very 

 obvious ; muscular impressions on the anterior side of the beaks 

 very distinct in the cast. 



Width of largest specimen j\ of an inch ; generally less. 



Orthis costata, n. s. Semicircular or semioval ; surface 

 marked by about twelve strong ribs ; area well defined, propor- 

 tionally large ; hinge-line scarcely equal to the width of the 

 shell, which is about j'^ of an inch. 



This may perhaps be a young shell, but I have seen no large 

 individual of the same species. The ribs are remarkably strong 

 for a species of Orthis. 



Besides the shells enumerated, there is a small Atrypa found 

 associated with them ; the same however is found in the Trenton 

 limestone in New York, and will be described elsewhere. Some 

 small Tentaculites and minute spiculas of some kind also occur 

 with these shells, but I have thus far been unable to refer them 

 to any known fossil. Mr. Carly also informed me that other fos- 

 sils, much more minute, occur in the marly clay, and it is not im- 

 probable that the microscope of Prof. Bailey may be called into 

 requisition to decide the characters of these ancient microscopic 

 beings. 



From the geological position of these minute fossil shells, and 

 from their great numbers, we may presume that they are widely 

 distributed, but are discoverable only under favorable circum- 

 stances. The minute species of Atrypa, which occurs with 

 them in considerable numbers, I have also found in the Trenton 

 limestone ; but it is scarcely probable that those here described, 

 which are still much more minute, would be detected at all. 

 They might even exist in considerable numbers in a compact 

 limestone, associated with fragments of corals, crinoidal joints, 

 and broken shells, without being detected by the naked eye. 



The occurrence of those here described is sufficiently inter- 

 esting to induce a careful examination in all our older strata for 

 minute fossil shells ; and since they are known in other forma- 

 tions and in a recent state, there seems no good reason why they 

 may not be found from the commencement of organic existence, 



Albany, December 10, 1844. 



