SUPPLEMENT. 
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of double laminae, deeply concave in the centre, numerous, thin, 
approximate, sinuous on the longest diameter ; siphuncle monili- 
form, ventral, consisting of a round tube, -which is exceedingly 
expanded between the septa, like the siphuncle of Ormoceras 
\_Actinoceras]. 
Gonioceras anceps. — a. Much weathered fragment, drawn from a specimen in 
the National Collection : natural size. b. Section, showing position of 
siphuncle ; copied from Hall, loc. cit. f. 1 c. 
“ This remarkable fossil, usually appearing upon the weathered 
surfaces of rocks, with the ventral or dorsal side exposed, and 
presenting a broad surface with extended septa and central siphon, 
has received the appellation of '‘petrified, fish’s bones.’ I had been 
disposed to regard this structure .... as due, in part at least, to 
pressure ; but the examination of numerous specimens in the rock, 
and of a portion of one nearly perfect, compels me to decide that 
their apparent disproportions are natural, and not the result of 
accident. In a portion of a nearly perfect individual, the siphuncle 
is so near the outer shell as to produce a longitudinal ventral ridge. 
