AMMONITID^. 69 



* 



Lytoceras and Phylloceras, Siiess, and Haploceras of Zittel, 

 combining characteristics whicli are found in all of these, besides 

 having peculiar characters of its own, and a different develop- 

 ment. The extent of involution is comparable with that of 

 Haploceras, but the whorl itself is about intermediate between 

 the extreme roundness of Lytoceras and the more flattened sides 

 of Phylloceras. Its peculiar characteristics consist in having 

 large lateral tubercles and exterior pilae, which are united as they 

 near the tubercles. The smooth zone along the outer centre in 

 the young is also probably of subgeneric value. G. Hyatti, Meek 

 (xxxvii, 14). Trias ; Nevada. Hyatt makes this a distinct genus 

 of his family Physanoidse, but Mojsisovics and Fischer class it 

 as a group of Ceratites. 



MEEKOCERAS, Hyatt, 18*79. Distinguished from Ceratites by 

 having but three distinct lateral cells and two lateral lobes, 

 besides the finer auxiliary lobes and cells. The t3'pical Ceratites 

 have at least four distinct lateral cells and lobes besides the 

 auxiliar^^ ones, and the distinction is slight between the two 

 series ; in this genus, on the contrar}^, the auxiliary series, when 

 present, is not divided from the third lateral cell by a distinct 

 lobe, as in Ceratites, and the aspect of the third lateral cell is 

 often like that of a Goniatites. The compressed whorls of all 

 the species is of course a characteristic which is obvious when 

 they are contrasted with typical Ceratites, as is also the absence, 

 or merely transient appearance, of heavy nodes and ribs, except 

 perhaps in the least involute species. G. aplanatum, White. Tri- 

 assic ; S. E. Idaho. 



Xenodiscus, Waagen. 



Distr. — X. pUcatus, Waagen (xxix, 91, 92). Productus. 

 Limestone ( Carboniferous) ; India. 



Shell flat discoidal, with compressed whorls, perfectly rounded 

 on the siphonal side ; umbilicus mostly large, and the whorls 

 generally only slightly embracing each other; surface smooth, 

 or with distant rounded folds which are thickest near the umbil- 

 ical margin of the whorls, or with numerous slight plications 

 which are straight on the sides of the whorls, and slightly turned 

 towards the front near the siphonal margin ; the sutures are very 

 simple ; the siphonal and two lateral lobes always well-developed, 

 also a sutural lobe generally, but auxiliary lobes are generally 

 absent. The lobes are furnished with a slight indentation at their 

 termination, the saddles rounded, entire, without an}^ indenta- 

 tion ; the internal sutures show only one large antisiphonal lobe, 

 which terminates in two long, sharp points ; the body-chamber 

 occupies not quite one whorl ; aperture simple. 



Trachyceras, Laube, 1869. 

 Distr. — 16 Triassic sp. T. bicrenatus, Hauer (xxxviii, 30, 31). 

 T. Whitneyi, Gabb (xxxvi, 81, 89). Trias; Nevada, California. 



