46 FOSSIL MOLLUSCA OF THE CHALK. 



I find it impossible to compare them properly, for the original specimen of A. undatus 

 which Mr. J. de C. Sowerby has had the kindness to lend me, is so much crushed, that its 

 real characters cannot be distinguished; and that name had better be omitted from our 

 lists. 



36. Ammonites Lewesiensis, Mantell. Plate XXI, fig. 1. 



Ammonites Lewesiensis, Mantell. Fossils of the South Downs, t. 22, fig. 2. 



— — D'Orbigny. Paleont. Franc. Ters. Cret., t. 102, figs. 1, 2? note 



t. 101. 



A. testa subinjtatd : anfractibus rotundatis, lateraliter sub-compressis, undato-costatis ; 

 dorso loBvi rotundato : umbilico parvo : aperturd ovatd : septorium lobis lateralibus 4 

 inequaliter trifidis. 



Shell inflated : whorls few, with their sides a little flattened, and crossed by about 

 twelve broad, radiating undulations, or indistinct ribs, which commence at the edge of the 

 umbilicus, and die out before reaching the rounded, plain back : umbilicus rather small, 

 with steep sides, allowing above one third of the inner whorls to be seen : aperture oval ; 

 septa with four unequally trifid lateral lobes, and a very narrow dorsal lobe. 



The young shell differs little from the adult ; it has the same plain, round back, but 

 the undulations are hardly visible till it has reached a diameter of 4 to 6 inches : in very 

 old shells, of 2 feet in diameter, the outer whorl becomes quite smooth. 



This is one of the largest species of Ammonite found in the Chalk, sometimes 

 exceeding two feet in diameter : the dimensions of the specimen figured are — diameter, 

 16 inches ; height of the last whorl, 6 inches ; width of the opening, 5 inches. 



Rather rare in the Grey Chalk near Lewes and Dover. 



Three species have been published under the name of A. Lewesiensis by Mantell, 

 Sowerby, and D'Orbigny. The description and figure given by the first are so imperfect 

 as to have made me almost despair of ascertaining what it represented, until Mr. Woodward 

 discovered Dr. Mantell's original specimen in the vaults of the British Museum. The 

 specimen is very imperfect, a part of the back on the younger side of the whorl being 

 worn off, and the whole being so much crushed as to reduce the size of the umbilicus 

 considerably, and to render the whorls flatter, and the back less round, than in perfect 

 specimens : nevertheless it is sufficient to enable us to recognise the species with certainty. 



A. Lewesiensis, when full grown, much resembles A. peramplus ; but it is flatter, and 

 the undulations are continued all across the side of the whorl, instead of stopping in the 

 middle of its side, as in A. peramplus. The young shells of the two species have no 

 resemblance, A. Lewesiensis being nearly smooth, and the other ornamented with nume- 

 rous ribs, which cross over the back. The form of the lateral lobes of the septa is nearly 

 the same in the two species ; but in A. Lewesiensis the dorsal lobe is very narrow, while 



