147 
AMMONITES fibulatus. 
TAB. CCCCVll.— fig. 2: 
Spec. Char. Depressed, ribed ; inner whorls 
almost wholly exposed ; sides of the whorls 
flattened, their inner margins plain ; ribs 
numerous, united in pairs by smooth spines ; 
aperture oblong. 
Syn. Ammonites armatus? Young and Bird , 
Geol of Yorkshire , p . 250, t, 13, f, 9. 
The remarkable flatness of this shelf, is encreased by 
the spines filling up at intervals, the concave space around 
the whorls ; there are now and then, ribs that pass over 
the front without uniting with spines, these ribs are 
more' numerous in the latter whorls, and even alternate 
with the spines ; each rib when it leaves the spine is 
found divided into two, so that there are a greater num- 
ber passing over the front, than exist upon the sides. 
Found at Whitby, where it is not of very rare occur- 
rence. The smooth spines distinguish it well from A. 
armatus of Min. Conch, tab. 95. but like it they are com- 
posed of solid shell, and when broken off, leave a flat- 
ended tubercle, compared, together with the ribs, by 
Young and Bird, to a button and loop, whence its name. 
