EXPLANATION OF PLATES 41. 42. 61 



transverse ribs of the outer shell are strengthened 

 by repeated intersections of the subjacent edges of 

 the transverse Plates. (Original.) 



Fig. 3. Back view of Ammonites sphsericus, from the 

 limestone of Derbyshire, shewing the position of 

 the siphuncle upon the dorsal margin, with its collar 

 advancing outwards between the two simple dorsal 

 lobes ; the lateral lobes are also simple and with- 

 out foliations, and pointed inwards. (Martin Pet. 

 Der. T. 7.) 



Fig. 4. Ammonites nodosus (Ceratites). This is one of 

 the species peculiar to the Muschel Kalk. The de- 

 scending lobes terminate in a few small denticula- 

 tions, pointed inwards, and the ascending saddles 

 are rounded outwards, after the normal character of 

 Ammonites. (Zeiten. Tab. II. Fig. 1. a.) 



Fig. 5. Back of A. Nodosus, shewing the dorsal lobes 

 pointed inwards, and the collar around the siphuncle 

 advancing outwards. No edges of the transverse 

 plates are placed beneath the dome-shaped Tuber- 

 cles; these derive sufficient strength from their 

 vaulted form. (Zeiten. Tab. II. Fig. 1. b.) 



Plate 41. V. I. p. 349. 



Ammonites giganteus, found in the Portland stone at 

 Tisbury in Wiltshire. This beautiful fossil is in the col- 

 lection of Miss Benett. The chambers are all void, and 

 the transverse Plates and Shell converted to Chalcedony. 

 (Original.) 



Plate 42. V. I. pp. 350, 351. Note. 

 Fig. 1. Cast of a single chamber of Nautilus hexagonus, 

 shewing the siaiple curvatures of the edges of the 

 transverse plates, and the place of the Siphuncle. 

 (Original.) 



