56 FOSSIL MOLLUSC A OF THE CHALK. 



Aptychus Portlocki, Sharpe. Plate XXIV, figs. 2, 3, and 4 (6 ?) 



A. bivalvis, transversim semi-ellipticus ; valvis imbricatis, utrinque concentrice sidcatis ; 

 sulcis distantibus ; plica media elevatd. 



Bivalve, the united valves forming, transversely, a semi-ellipse, with a uniform curve 

 to the outer margin ; valves thin, marked on both sides by rather distant concentric 

 furrows, produced by the terminations of the broad imbricated additions to their outer 

 edges : medial fold large, and separated from the rest of the valve by a deep furrow. 



Height, f- inch ; breadth of each valve, ^ inch. 



Found in the Upper Chalk, near Norwich, by Mr. Bayfield ; and in a Flint Pebble, in 

 the Gravel, near Croydon. 



Of the Aptychi yet met with in the Upper Chalk, this species has the transverse 

 diameter of its united valves greatest in proportion to its height ; and Ammonites Portlocki 

 is the only species known in the Upper or Middle Chalk which has the diameter of its 

 mouth greater than its height (see Plate XXIV, fig. 12). Moreover, the curves of the 

 respective outlines of the two nearly correspond, and both are found in the Upper Chalk, 

 near Norwich. The probability of this Aptychus belonging to A. Portlocki is therefore 

 very strong. 



One of the three forms figured by Geinitz under the name of Aptychus cretaceus 

 (' Charakteristik, &c, des Sachsischen Kreidegebirges,' t. 17, fig. 25 a) nearly resembles 

 A. Portlocki in outline, but, being a fragment, its proportions are not shown ; and it is 

 distinguished by its closely set imbrications from our species. 



Anatifa convexa, Roemer, ' Kreidegeb.,' t. 16, fig. 7, is the plate of an Aptychus 

 closely approaching A. Portlocki ; but his figure and description do not enable me to 

 identify them. 



Aptychus Gollevillensis, Sharpe. Plate XXIV, fig. 5. 



A. bivalvis, truncato-ovatus ; valvis imbricatis, utrinque concentrice sulcatis ; sulcis 

 approximatis : plied media elevatd. 



Bivalve, truncato-ovate, the united valves forming about two thirds of the shape of an 

 egg ; very convex, especially towards the outer edges : valves thin, marked on both sides 

 by closely set concentric furrows, produced by the terminations of the narrow imbricated 

 additions to the outer edges: medial fold elevated, and sharply separated from the rest of 

 the valve by a deep furrow. 



