252 
AUG. F. FOERSTE 
half of the living chamber, follo’wed by a slightly concave length- 
wise curvature along the upper half of this living chamber, in a 
normal shell, somewhat as in Oncoceras plebeium Hall, from the 
same locality and horizon as the type of Oncoceras lycum. 
The specimen is a cast of the interior of the conch. In this 
specimen the cast of the interior of the living chamber is con- 
tracted abruptly and strongly near mid-height. The contraction 
begins about 8 mm. above the base of this chamber and reaches 
its maximum at 10 mm. above this base; above this level of 
maximum contraction the cast expands evenly as far as the 
aperture. The dorso-ventral diameter of this cast, at the base 
of the living chamber, is 17 mm.; 8 mm. above the base it is 
18 mm.; at the most narrowly contracted part of the chamber 
it is 15 mm. ; and from this point of greatest contraction the cast 
of the interior enlarges evenly to approximately 17 mm. at the 
aperture, as far as can be determined from that part of the aper- 
ture which is preserved. This abrupt and strong contraction 
of the cast of the interior of the living chamber may not be indi- 
cative of any corresponding contraction of the exterior of the 
shell. On the contrary, it may be confined to its interior, and 
may be due to a very gerontic thickening of the interior of the 
living chamber, at mid-height, in case of some individual speci- 
men; or, the animal after death may have contracted along the 
upper half of the living chamber, as appears to have been the 
case not infrequently in some Silurian Poterioceroids. The sym- 
metrical contraction of this cast favors the first one of these 
views, namely that the walls of the conch were thickened above 
mid-height by a gerontic deposit upon their interior surface. If 
this was the case, then the exterior of the living chamber may 
have contracted toward its aperture very much as in Oncoceras 
plebeium, from the same horizon and locality. In fact, Oncoceras 
lycum may have differed from the type of Oncoceras plebeium 
chiefly in its smaller curvature lengthwise, and in the greater 
height of the camerae. 
The phragmacone enlarges from a dorso-ventral diameter of 
11 mm. at its base to 17 mm. at its top; the length of the phrag- 
macone,- measured along its ventral side, is 22 mm. According 
to this rate of tapering, the total length of the conch probably 
did not exceed 75 mm., when measured along its ventral side. 
The lateral diameter of the specimen at its base is 9.8 mm., and 
