APTYCHUS. 55 



has a sharp keel, and may be excluded from the comparison, as it is too small to admit 

 any of the Aptychi yet found. 



As yet we have only seven forms of Aptychus from the Chalk, six of which have so 

 many characters in common, that it is natural to suppose that they belong to Ammonites 

 of the same, or nearly allied groups ; the remaining species, A. ruyosus, differs in having 

 the convex side strongly ribbed. 



In Plate XXIV, which is devoted to this subject, are represented all the known 

 Aptychi of the Chalk, and outlines of the openings of the various Ammonites to which it 

 seems probable that the Aptychi may belong ; and, to facilitate the comparison, a dotted 

 line is drawn across each opening. It must be remembered that the two plates of the 

 Aptychi did not lie flat in the same plane, but were slightly inclined towards one another 

 [on the inner side], in which position the width they occupied would be rather less than 

 when they are represented lying side by side on the plate. 



Among the Aptychi belonging to the Chalk, figured by Continental authors, Aptychus 

 cretaceus, of Munster, so nearly approaches several of those which are here described, as 

 to show that they belong to nearly allied Ammonites. But sad confusion has been made 

 with that specific name. Geinitz gives three figures of Aptychus cretaceus (' Charakteristik 

 des Sachsischen Kreidegebirges,' t. 17, figs. 25 a and b and 20), which evidently belong 

 to three different species ; another distinct species is figured under the same name by 

 Reuss (' Versteinerungen der Bohmischen Kreideformation,' t. 7, fig. 13), and both these 

 authors refer to Aptychus cretaceus, the plate described under the name of Anatifa convexa, 

 by Roemer ('Kreidegebirges,' t. 10, fig. 7), which, if the figures are to be trusted, differs 

 from all the others. 



Aptychus leptophyllus, Sharpe. Plate XXIV, fig. 1 a, b. 



A. bivalvis, subtruncato-ovatus ; valvis imbricatis, latere convexo Icevi, latere concavo 

 concentrice lineato ; plica media inconsjneud. 



Bivalve, thin, the united valves ovate, with a slight truncation at the lower end ; outer 

 edge reflected : the convex side smooth ; the concave side delicately marked by very close, 

 concentric imbrications : medial fold narrow, and but little elevated. 



Height, 1| inch ; breadth of each valve, ^ inch. 



Found in the Upper Chalk, near Brighton, by Mr. Morris. 



The form of this Aptychus corresponds to that of the opening of Ammonites leptophyl/u.s, 

 and which is also found in the Upper Chalk, near Brighton ; for these reasons it is probable 

 that it belongs to that species. 



