286 
AUG. F. FOERSTE 
the diameter of the conch, increasing to 7 camerae toward the 
larger end of the specimen. The surface of the shell is chiefly 
smooth, but 2 or 3 mm. below the sutures of the septa there is 
in several places a transverse ridge curving downward with the 
sutures on the ventral side of the conch, so as to suggest the 
former presence of a hyponomic sinus. The amount of this 
downward curvature is at least 5 mm. near the larger end of the 
specimen. It is approximately commensurate to that of the 
Actinoceras from the Boothia Felix-King William Land area, 
represented in this paper by figure 2 on Plate XXX. 
Figured specimen. — Figure 2 on Plate XXVIII of this paper 
represents a typical specimen of Actinoceras tenuifilum from the 
Black River at Watertown, New York. It is part of the dupli- 
cate material originally belonging to the New York State Museum 
of Natural History, and was presented to the writer by Dr. 
Rudolf Ruedemann. About 5 camerae occupy a length equal 
to the diameter of the conch at the smaller end of the specimen, 
increasing to 5.5 camerae at its upper end. The diameter of the 
siphuncle equals about 64 per cent of that of the conch, and its 
margin is within 1 mm. of the ventral wall of the latter. The 
septa are strongly concave. The septal necks descend slightly 
over 2 mm. below the general concave curvature of the septa, 
and the distance between successive annulations of the siphuncle 
equals or slightly exceeds 2 mm. The septa are in contact with 
the lower surface of these annulations. Toward the larger end 
of the specimen, the annulations of the siphuncle decrease in 
size as in Par actinoceras. The sutures curve downward toward 
the median part of the ventral side a distance of 4 or 5 mm. 
At the base of the siphuncle, its axial part, for a diameter of 
about 2 mm., is free from organic deposits, being filled with 
black matrix. Toward the top of the siphuncle this axial part 
free of organic deposits increases more or less irregularly to 5 
mm. in diameter, indicating progressive deposition of organic 
material with increasing age. 
Resume. — In typical Actinoceras tenuifilum the conch is slight 
' depressed dorso-ventrally, the apical angle is about 12 degrees; 
the number of camerae in a length equal to the diameter of the 
