AMMONITES. 53 



45. Ammonites Wiltonensis, Sharpe. Plate XXIII, fig. 10 a — c. 



\_A. testa compressd, transversim costatd, costis parvis, numerosis, obtusis, incequalibus ; 

 aperturd elongatd, antice rotundatd ; umbilico parvo. Lat. maj., 13 ; rnin., 11 ; alt., G lines. 



This figure is taken from a small Ammonite in hard, gritty Chalk, with green stains, 

 stated to have been found near Devizes, by Mr. W. Cunnington. The form is unusual 

 in the Chalk ; the ribs are simple, and sub-equal on the back of the shell, but many 

 become obsolete on the sides, the remainder becoming stronger at the umbilicus ; the 

 sutural lobes are few and rather simple.] 



46. Ammonites Jukesii, Sharpe. Plate XXIII, fig. 11 a — e. 



\_A. testa discoided, late umbilicatd, transversim costatd; costis numerosis, acutis ; an- 

 fractibiis sub-compressis, interdum constrictis ; aperturd ovatd. 



This specimen figured is the fragment of an Ammonite which, when perfect, probably 

 measured 5 or 6 inches in diameter ; it consists of about one third of the external volution 

 and portions of two inner whorls, which have been detached, and represented separately 

 (fig. 11 b, c). The curvature of the ribs is convex towards the aperture; but the figures 

 are not only reversed, but placed upside down. The outer whorl shows one of the 

 periodic constrictions, and another is seen on the inner whorl when detached, and also a 

 more prominent rib on the posterior side of the constriction. 



Ammonites Jukesii was discovered by Colonel Portlock, R.E., in the hard Chalk of the 

 county of Londonderry.] 



Genus — Aptychus, H. Von Meyer. 1821.* 



The operculum [or calcarious plates presumed to have closed the aperture] of certain 

 species of Ammonites, of which there are three forms distinguished by Voltz : 



1. Cornet, consisting of a single horny plate, of a semi-circular, semi-elliptic, or 

 truncato- ovate form, marked by concentric wrinkles, with a semi-circular notch on the 

 middle of the straight side. 



2. Imbricati, consisting of a pair of equal plates, each a quarter of a circle, or quarter 



* The name Trigonellites, given by Parkinson, who described and figured several forms in 1811 ('Org. 

 Rem.,' hi, 18-1), ought to have been employed. Meyer added nothing to our knowledge respecting them. 



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