112 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



angle has a distinct spine projecting obliquely beyond the margin. 

 The hinge-line is canaliculate on the back, and the margins of 

 the valves project in a thin sharp line along their junction at the 

 ends, and less conspicuously on the ventral side. 

 Surface marked by distinct undulating striae. 



This species lias all the external marks of Leperditia ; and the striae, 

 or impressions of the vessels radiating from the central tubercle, are very 

 distinct under a lens. 



This shell is slightly smaller than L. punctulifera of the same rocks. 



LEPERDITIA SENECA (n.s.). 



In the same shale with L. spimdifera are several specimens of a short 

 ovoid form of Leperditia, which is essentially smooth, and for the most 

 part the individuals are smaller. A larger specimen of apparently this 

 species, where the crust is removed, shows an indentation on the centre of 

 the valve. An impression of another individual has the appearance of having 

 been made by a granulose surface. 



Of the three species known in the Hamilton group, the L. pujicUilifera 

 is far the most abundant. 



Geological formation and locality. In the shales of the Hamilton group : 

 Ontario county, N.Y. 



ANNELIDA. 

 GENUS SPIRORBIS (Lamarck). 

 SPIRORBIS ANGULATUS ( n. s.) 

 Discoid or very slightly ascending, making two or more volutions : 

 outer volution robust; the transverse diameter greater than the 

 dorso-ventral, and the sides sometimes subangular. Surface la- 

 mellose striate, the lamellae undulating and sometimes crowded 

 into ridges, and the upper angular side sometimes nodose. The 

 aperture is rounded or oval, and usually nearly rectangular to the 

 plane of volution, but sometimes turned upwards. 

 This species is usually attached to shells, and particularly to Tropido- 

 LEPTUS. It is a mose robust species than the S. laxus, and less distinctly 

 annulated. 



Geological formation and locality. In the shales of the Hamilton group : 

 at Darien in Erie county, and in Ontario county, N.Y. 



[ September 1861. 



