110 MOLLUSCA OF THE GREAT OOLITE OF YORKSHIRE. 



A. Testa discoided, subinflatd ; anfractibus involutis subcompressis, rotundatis, lateribm 

 26 30 costatis ; costis subrotundatis, obtusis, in media laterum hi vel trifurcatis continuis ; 

 aperturd semilunari, umbilico angmtato. 



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 BLAGDENI, Sow. Plate XIV, fig. 3, b. 



AMMONITES BLAGDENI, Sow. 1813. Min. Con., pi. 201. 



CORONATES, Schloth. 1813. (Not Am. coronatus, Brug., 1789.) 



Zieten. 1830. Pet. Wurtemb., t. i, fig. 1. 

 BLAGDENI, Phil. 1835. Geol. of Yorksh., vol. i, p. 124. 

 CORONATUS, Quenstedt. 1843. Floz. Wurtemb., p. 326. 

 BLAGDENI, If Orb. Ter. Jurrass., p. 396, t. 132. 



A. Testa discoided, subcylindricd, late umbilicatd; anfractibus subdepressis, lateribus 

 declivibus, costatis ; costis 20 28 externe tuber culatis, subacutis ; dor so 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 costse, terminating in spiniform tubercles 

 on the outer margin, and from each of which arise 3 to 5 smaller costse, 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 1 7 marginal 

 ribs ; in another specimen from the same locality (Scarborough), about one foot in diameter, 

 the number is 28. The numerical proportion of these costse, 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 costae, 

 showing an increase; in the other specimen, the inner whorl has 21, and the outer only 

 17 costse, showing a decrease in their number. The specimen figured by M. D'Orbigny 

 has only 15 tubercular costae surrounding the umbilicus. 



Locality. Near Scarborough ; Inferior Oolite, Somerset ; Bayeux, &c., France ; Brown, 

 Jura S, Stuiffien, Wurtemberg. (Quenstedt.) 



