AEGOCERAS PLANICOSTA. 337 



and by the rounding of the abdominal surface of this region of the shell. A comparison 

 of d'Orbigny's PI. 103, figs. 1 and 2, with our PL XXIV, fig. 5, will convince the student 

 of the identity of the French and English forms. 



In describing this species it will contribute to a more correct knowledge of its forms if 

 we trace them through the four stages over which they pass, and which I shall call — 1st, 

 the planicostal, 2nd, the ziphus, 3rd, the Dudressieri, and 4th, the adult condition. The 

 planicostal stage extends to five whorls ; the two first whorls are smooth and slowly 

 acquire ribs which, in the other three, are slender and prominent, and as they bend over 

 the siphonal area they become much flattened and are separated by concave spaces as 

 wide as the flat portion of the ribs. All the whorls are fully exposed, and the umbilicus 

 is widely open through the slight involution of the encircling whorls. 



The ziphus stage commences with the sixth whorl, fig. 1 . The ribs are now more promi- 

 nent, and before they bend over the siphonal area a sharp prominent tubercle is developed 

 on the termination of the rib, which gives a considerable extension to the area, and leaves 

 the prominence of the large transverse elevations in the direction of the ribs, on the surface 

 of which are ornate lines of delicate sculpture (see PI. XXIV, figs. 4, 6, 7). 



In the Dudressieri stage (fig. 5) the shell is discoidal, thick, and compressed, with a 

 wide open umbilicus and very evolute spire exposing the entire whorls to view. Here we 

 count seven volutions which, when carefully examined, show the different characters already 

 pointed out in the description of the morphology of this species, and which are very well 

 shown in figs. 1 and 5 of PI. XXIV. 



The adult state is not often seen ; most of the large shells of this species I have examined 

 had come to grief, partly in consequence of the slender involution of the spire. Fortu- 

 nately, however, I found a nearly perfect example in the Jermyn Street Museum (School 

 of Mines), obtained from Lyme Regis, which afi'orded a rare opportunity for studying the 

 adult condition of Aeg.planicosta. No person could be expected to believe the specimen was 

 this species unless he had followed the changes of shape through which this fine Ammonite 

 passed. The fossil afforded not only a good example of a generalised form of a remark- 

 able species, but at the same time showed how persistent and well-defined the different mor- 

 phological changes were, which allied species of Ammonites endured amongst the various 

 transition forms through which they were destined to pass between youth, adolescence, 

 and old age. Here we discover no confusion with allied species, each holds its own predes- 

 tined course during the varied evolutionary stages through which it has to pass, so that 

 amongst Ammonites we fail to detect any transmutation of specific forms one into another: on 

 the contrary, each obeys its own law of successive change between the ovum and maturity. 

 PI. XXV shows Aegoceras planicosta, Sow., of its natural size and in its adult condition, and 

 represents a large Ammonites Dudressieri, d'Orbigny. The sides are ornamented with forty 

 ribs, which are straight, regular, and prominent, with equal sulcations between ; the penul- 

 timate has the terminal tubercles on the ribs as in Dudressieri, but in the last whorl 

 we observe the disappearance of the tubercles, and the appearance of a number of trans- 



44 



