62 



nana Grabau I var. ). The Nautilus is of especial interest 

 from the large size of the specimens obtained, which 

 often measure six or eight inches in greatest diameter. 

 The specimens occur in the concretionary masses of which 

 this bed is composed, and from the nature of the rock and 

 the brittleness of the fossils, great care is needed in freeing 

 them from the matrix. This is by far the largest and finest 

 fossil found in the region, and its restriction to this, the 

 lowest bed of the Hamilton shales proper, is of great 

 interest. This bed has therefore been named the Nautilus 

 bed.* It was from this bed that the original specimens 

 described and figured in Volume V. of the Palaeontology of 

 New York, were obtained, and so far as known, this species 

 has not been found elsewhere. 



Ambocoelia umhonata var. nana is a small representa- 

 tive of the species, the brachial valve differing in its greater 

 convexity and its general resemblance to that of Spirifer 

 subumbonus Hall. The proportions differ from the normal 

 for the species, and the surface is marked by numerous 

 elongated pits, as in A. spinosa Clarke. This variety 

 occurs in great numbers, and characterizes the rock where- 

 ever found. 



Another fossil which is practically restricted to this bed, 

 and which occurs in great numbers, associated with the 

 preceding, is Camarotoechia dotis Hall. These shells are 

 usually found in an excellent state of preservation, and 

 where differential weathering has left them in relief, details 

 of structure appear clearly. Sometimes the rock is 

 made up of these shells and the little Ambocoelia, and 

 in such instances it is a comparatively pure limestone, 

 though ordinarily it partakes more of the nature of a 

 calcareous clavstone. Other fossils are extremelv rare, and 

 occur mainly in the shaW portions of the bed. 



•Faunas of the Hamilton Group, etc. 





