•558 G. C. Crich — On Ammonites calcar. 



Oa the whole we think there cannot be any doubt that Zieten's 

 Amm. calcar is only an example of his Amm. bipartitus, deformed iu 

 such a manner that the marginal tubercles of the periphery have 

 coalesced into a single row of tubercles occupying nearly the median 

 line of the periphery, just as in the example of HopUtes tuberculatus 

 from the Gault of Folkestone, described by the present writer in 

 a previous number (December. 1898) of this Magazine. There are 

 individual differences in Amm. bipartitus, among which may be 

 noticed a variation in the prominence of the ridge on the middle 

 •of the lateral area, in the number of the marginal tubercles, and 

 in the coarseness of the ornaments. Amm. calcar agrees with an 

 average specimen. 



Having decided that the type-specimen of Zieten's Amm. calcar is 

 a deformed example of his Amm. bipartitus, a few words must be 

 added respecting the name of the fossil. 



Firstly, in regard to the trivial name. Both Bronn ^ and OppeP 

 considered Zieten's Amm. bipartitus to be a synonym of Stahl's 

 Amm. bicostutus,^ and we think there can be no doubt as to the 

 correctness of this identification. Quenstedt* refers Zieten's Amm. 

 bipartitus and Stahl's Amm. bicostatiis to the same species, for which, 

 however, he adopts the name Ammonites bipartitus. But since 

 Stahl's name antedates Zieten's by some seven years it should 

 stand, and the trivial name of the fossil should be bicostatus. 



Secondly, in regard to the generic name. There is no difficulty 

 about the generic name of this species, for Munier-Chalmas ^ selected 

 Ammonites bipartitus, Zieten, as the type of a new genus, for which 

 'he proposed, in 1892, the name Disticlioceras, without, however, 

 giving any diagnosis of the genus. This was subsequently supplied 

 by Parona & Bonarelli,^ who gave not only a diagnosis of the 

 genus, but also a list of the forms which they included in it. 

 Cossmann," however, points out that this name is preoccupied by 

 Distichocera, of Kirby, and accordingly alters it to Bonarellia. 



Hence the British Museum example of Zieten's Ammonites calcar, 

 which is most probably the type-specimen, is merely a deformed 

 example of Bonarellia bicostata, Stahl, sp. 



1 H. G. Broun : Index pslajont., 1848, p. 33. 



3 A. Oppel: "Die Juraformation," 1856-58, p. 560 (1857). 



3 G. Stahl : Corresp.-Blatt Wiirt. Laudwirtlisch. Ver., Bd. vi (1824), p. 49, fig. 9. 

 Stahl's description is as follows : — " Er liat eine vertiefte Eiickenlinie, deren Seiten 

 uach aussen mit kleinen Zahnen besitzt siud ; von jedem Zahn gehen zwei Streifen 

 bis an die in der Mitte der Windungen hinlaiifende Streifungslinie, von vro an die 

 andere Halfte der "Windung glatt ist. Er findet sich verkiest bei Gamnielshausen im 

 jungera bituminiisen Mergelschiefer ; erfordert iibrigens noch nabere Untersucbung." 



* F. A. Quenstedt : Amm. Scbwab. Jura, Bd. ii (1887), p. 732. 



5 Mimier-Chalmas: Bull. Soc. Geol. France, ser. 3, vol. xx, No. 6, Comp. rend., 

 p. clxxi, 1892. 



^ C. F. Parona & G. Bonarelli : Mem. Acad. Sci. Savoie, ser. 4, vol. vi, p. 139, 

 1897. 



' M. Cossmann : Rev. crit. de PaleozooL, vol. ii, p. 75, 1898. 



