74 REPORT — 1864. 



adult condition, and proceeded to ajiply this law to the development of the shell of 

 Ammonites. From an extensive series of specimens he had collected from the Lias 

 and Oolitic formations, he could show, 1st, that one group of species exhibit very- 

 little change in their various phases of growth ; and 2nd, that another gi-oup ex- 

 hibits such diverse changes that their several sta2:es of development have been 

 assumed as permanent forms, and described as distinct species. 



To the first group belong the following species from the Lias fonnation. 



Lo-WEB Lias. 



Ammonites BucUandi, Sow, Ammmites BircMi, Sow. 



obtusus, Sow. Sauzeanm, d'Orbig. 



Conybcari, Sow. ■ raricostatus, Ziet. 



Botmardi, d'Orbig. 



ISIlddle Lias. 



Ammonites ibex, Quenst. Ammonites Guibnlianm, d'Orbig. 



bipunctalKs, Rcemer. miiriv, Ziet. 



Loscombi, Sow. Davm, Sow. 



Bccheii, Sow. striatus, Renecke. 



Jimbriaius, Sow. 



Upper Lias. 



Ammonites bifrotis, Brug. Ammonites Hollandrei, d'Orbig. 



communis, Sow. comphmatm, Brug. 



annulatus, Sow. hircimis, Ziet. 



Jibidattis, Sow. jurensis, Ziet. 



The second group in which important and varied changes take place are 



LowEK Lias. 



Ammonites planorbis, Sow. Ammonites semicostatus, Y. & B. 



am/ulatus, Schloth. bifer, Quenst. 



planicostatus, Sow. 



Middle Lias. 



Ammonites Jamesoni, Sow. Ammonites caprieoi'nus, Schloth. 



The author gave a detailed description of the moi-phological changes exhibited 

 by each of these species, and demonstrated that Ammonites planicosttitus, Sow. was 

 the young shell oi Am. Dudrcssieri, d'Orbig. This species acquired spines on the 

 dorsal border of the ribs in the second phase of its growth, wliich became tubercles 

 in a third stage, and these it finally lost as it advanced to maturity. Am. Jamesoni, 

 Sow., was shown to be the adult form of A. Bromiii, llrem., with tubercles on the 

 ribs, and a rudimentary keel ; in a second stage of growth it became Am. Megnardi, 

 d'Orbig., and afterwards changed to the elegant form figured by Sowerby. Am. ca- 

 pricornits in its different phases of development had been the tj'pe of six figured 

 species : in early age it was Am. maculatits, Y. & B. ; a little older it was Am. plani- 

 costa, d'Orbig., and Am. latcecostus, Sow. ; in middle age, when the last whorl ex- 

 panded somewhat abruptly, and supported two rows of small tubercles on the late- 

 ral ribs, it formed the Am. hetenn/enes, Y. & B. ; and two-thirds of the last whorl in 

 this stage of growth was figured by Sowerby as Am. Hcnltyi, Sow. 



These facts were demonstrated by a series of specimens exhibiting the morpho- 

 logical characters of the dift'erent species described, and the important practical 

 bearing of the subject on Palajoutology was dwelt upon. Ammonites were now 

 generally admitted to be the best indicators of the stratigraphical position of the 

 dift'erent zones of life in the secondary rocks, and it was therefore all tlie more im- 

 portant to geology that the species of this group should be rigorously detennined ; 

 which coidd only be done by a critical examination of tlieir morphological charac- 

 ters ; for mere species-making, without such knowledge, was hindrance, and not 

 progi'ess, in the present state of Palaeontology. 



