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, showing 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 without 

 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, showing 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 vault- 

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



Plate 41. V. I. p. 264. 



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 Calcedony. 

 (Original.) 



Plate 42. V. I. pp. 264, 265. Note. 



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

 showing the simple curvatures of the edges of the 

 transverse plates, and the place of the Siphuncle. 

 (Original.) 



vol. ii. 6 



