54 FOSSIL MOLLUSCA OF THE CHALK. 



of an ellipse, in form, marked on one or both sides by strong concentric lines or wrinkles, 

 produced by the imbricated growth of the shell ; each plate has a fold along its straight 

 side, the fold of one plate lapping over that of the other : the plates usually thin. 



3. Cellulosi, consisting of a pair of plates, similar in form to those of the Imbricati, 

 and meeting in the same manner, but thick, and with the convex side cellular in structure. 



Few organic remains have given rise to more different opinions than these curious 

 bodies. [Professor Quenstedt regards them as the opercula of Ammonites, a view which 

 has been generally adopted in Germany. See Woodward's 'Manual,' 1851, p. 80, and 

 Owen's 'Lectures on the Invertebrata,' 2d edit., 1855, p. 596. In Part I, p. 19, of this 

 Monograph, the concave side of the Aptychus is incorrectly stated to have been external 

 instead of the convex side.] 



All the Aptychi yet found in the Chalk belong to the group of the Imbricati ; they 

 have all very thin shells, owing to which they are seldom found perfect ; and they are all 

 from the Upper or Middle Chalk.* As there is now little doubt that these bodies really 

 belong to some part of Ammonites, I have endeavoured to assign them to their respective 

 species of Ammonites, in which I have been guided by a comparison of the form and size 

 of the Aptychi with the apertures of the Ammonites found in the same beds of the Chalk 

 Formation, and in the same localities, for I am not aware that any Ammonite has yet been 

 found in the Chalk containing an Aptychus within it. As there are but few species of 

 Ammonites known in the Upper and Middle Chalk, the comparison is more satisfactory 

 than might have been anticipated ; but the result is only conjectural, as the discovery of 

 other species of Aptychus or of Ammonites might materially alter our conclusions on this 

 subject. 



The species of Ammonites known in the Upper and Middle Chalk are A. Griffithii and 

 A. Oldhamii, only known in Ireland, whence I have seen no Aptychi ; for which they are 

 excluded from the comparison. 



A. Gollevillensis* found in the Upper Chalk in Ireland and the North of France, and 

 which may consequently be expected to occur here. 

 A. Portloc/ci*) 



A. Icenicus* >in the Upper Chalk of Norwich. 

 A. Felleda* J 



A. leptophyllus* in the Upper Chalk of Kent and Sussex. 

 A. peramplus* 1 



A. Woolgari, /in the Middle Chalk of our Southern Counties. 

 A. BravaisianusJ 



The species marked * have round backs, and belong to Von Buch's family of Ligati. 

 . /. Woolgari has a serrated keel, and belongs to the Rothomagenses ; A. Bravaisianus 



\ir. Flower has also found two species in Flint Pebbles in the Gravel, near Croydon. 



