402 



On '^ Orthoceras [Endoceras] duplex." 



the diameters being as 24 : 19. Very uniformly tapering at 

 the rate of 1 in 6. Septa direct, undulating ; distant about 

 ^ th'C diameter. Siphuncle eccentric, cylindrical, its diameter 

 about I the longer diameter of the shell. Body-chamber and 

 test unknown. 



Hematics. The distinguishing feature of this species (fig. 3) 

 is its relatively high rate of tapering. 



Horizon. Ori/^oceras-Limestone (=Arenig-Llanvirn). 



Locality. Reval (Esthoniaj, Russia. 



Fio'. 3. 



Orthoceras revahnse. — a, part of a specimen, the whole of which measures 

 71 inches (s, siphuncle) ; b, section, showing siphuncle at s. Natu- 

 ral size. 



