AMMONITKS. ^^0 



K. Dorsal keel crenated. 

 1.-). Amalthei. Jura. A. cokdatus, Sowb. PI. 103, ligs. r.CI, .^02. 

 Ki. liotliomagenses. Clialk. A. rothomagensis, Brong. PL 10:'., fig. 50:^,. 



F. Dor sally sharp edged. 

 17. Disci. Chalk. A. Mettehnichii, Hauev. PI. 103, fig. r^U. 



L. von Buch has attempted to distinguish a large portion of 

 the above groups by differences in the lobation {Ahh. Akad. 

 Berlin^ 1830), and d'Orbigny has further modified them. 



Amongst the various attempts which have been made to 

 ^' bring order out of chaos "in the arrangement of the Ammo- 

 nites, that of Prof. Alpheus Hyatt deserves particular mention. 

 In his article on "Fossil Oephalopods," published in the Bulletin 

 of the 3Ius€i(m of Comparative Zoolor/i/, i, 71, this author regards 

 the Ammonoids, including all the Cephalopods with serrated 

 or foliated septa, the Clymeniiv, ({cmiatites. keratites, and Am- 

 monites proper "as a distinct order from the Nautiloids and 

 Dibranchiate Cephalopods ;'' the typical group of this order 

 being the so-called genus Ammonites. This enlarged view of 

 the systematic position of the Ammonoids is by Prof. Hyatt 

 attributed to Prof. Agassiz, but it is evident that Yon Buch had 

 a glimmering of the same idea because his groups (mainly those 

 I have enumerated above) although permitted by him to remain 

 under the generic name Ammonites were designated as "families." 

 Prof, Edward Suess, also, regarded the genus Ammonites as a 

 family, the typical groups of which were of generic rank. 



I give below the diagnoses of the families and genera in Prof. 

 Hyatt's paper (which includes only liassic forms), premising that 

 whilst the discoveries of the embryonic differences between the 

 Nautiloids and Ammonoids made by Prof. Hyatt are supposed b^- 

 some to indicate that the latter should be included among the 

 dibranchiate rather than among the tetrabranchiate cephalopods, 

 in any event, the elaborate subdivisions of the group are scarcely 

 warranted by the very changeable characters of the species. 

 Regarded as a convenience simpl}', the modified arrangement 

 of Von Buch, which we have given, appears preferable. 



Prof. Hyatt reverses the use of " dorsal " and " abdominal " 

 in his descriptions of the shells ; inasmuch as the animal of 

 Nautilus and Ammonites is placed with its abdominal side to the 

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