110 MOLLUSCA OF THE GREAT OOLITE OF YORKSHIRE. 
A. Testa discoided, subinflatd ; anfractibus involutis subcompressis, rotundatis, lateribus 
26—30 costatis ; costis subrotundatis, obtusis, in medio laterum bi vel trifurcatis continuis ; 
aperturd semilunari, umbilico angustato. 
A discoidal, somewhat inflated, shell, with rather subcompressed volutions, and a narrow 
and deep umbilicus; margin of the umbilicus with 26 to 30 obtusely-rounded ribs, which 
subdivide into two or three smaller ones in passing over the back; aperture semilunar, 
deeply impressed by the previous volution, 
Locality. Near Scarborough. 
Ammonites Buacpent, Sow. Plate XIV, fig. 3a, 6. 
AMMONITES BLAGDENTI, Sow. 1813. Min. Con., pl. 201. 
— coronatus, Schloth. 1813. (Not Am. coronatus, Brug., 1789.) 
— — Zieten. 1830. Pet. Wurtemb., t. i, fig. 1. 
— BiaGpENI, Phil. 1835. Geol. of Yorksh., vol. i, ps 124. 
— CORONATUS, Quenstedt. 1843. Floz. Wirtemb., p. 326. 
— Biacpent, D’Orb. Ter. Jurrass., p. 396, t. 132. 
A. Testdé discoided, subcylindricd, late umbilicatd ; anfractibus subdepressis, lateribus 
declivibus, costatis ; costis 20—28 externé tuberculatis, subacutis ; dorso subconvexo, trans- 
versim costato ; aperturd transversd, quadrangulari. 
A discoidal, thick, and widely-umbilicated shell, formed of rather depressed quadran- 
gular volutions, ornamented with 20 to 28 obtuse costs, terminating in spiniform tubercles 
on the outer margin, and from each of which arise 3 to 5 smaller costa, which pass over 
the back ; the aperture is transverse and quadrangular. 
In some specimens the tubercles are sharper, differently formed, and more numerous 
than in others. 
In the shell figured, which measures about six inches diameter, there are 17 marginal 
ribs; in another specimen from the same locality (Scarborough), about one foot in diameter, 
the number is 28. The numerical proportion of these coste, however, do not always 
increase or decrease with regularity during the progress of growth. ‘There are two 
specimens of Am. coronatus, Zieten, in the British Museum, which are certainly identical 
with our shell, in one of which the inner volution has 25, and the outer 27, marginal coste, 
showing an increase; in the other specimen, the inner whorl has 21, and the outer only 
17 costee, showing a decrease in their number. The specimen figured by M. D’Orbigny 
has only 15 tubercular costee surrounding the umbilicus. 
Locality. Near Scarborough ; Inferior Oolite, Somerset ; Bayeux, &c., France ; Brown, 
‘Jura 6, Stuiffen, Wurtemberg. (Quenstedt.) 
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