AMMONITIDA. 69 
Lytoceras and Phylloceras, Suess, and Haploceras of Zittel, 
combining characteristics which 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 pile, which are united as they 
near the tubercles. The smooth zone along the outer centre in 
the young is also probably of subgeneric value. C. Hyatti, Meek 
(xxxvii, 14). Trias; Nevada. Hy: att makes this a distinct genus 
of his family Physanoide, but Mojsisovics and Fischer class it 
as a group of Ceratites. 
MEEKOCERAS, Hyatt, 1879. Distinguished from Ceratites by 
having but three distinct lateral cells and two lateral lobes, 
besides the finer auxiliary lobes and cells. The typical Cer atites 
have at least four distinct lateral cells and lobes besides the 
auxiliary ones, and the distinction is slight between the two 
series ; in this genus, on the contrary, 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. C. aplanatum, White. 'Tri- 
assic; 8. E. Idaho. 
XeENopiscus, Waagen. 
Disir.—X. plicatus, 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 slig ehtly 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 any indenta- 
tion; the interral 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.bierenatus, Hauer (xxxvili, 30, 31). 
T. Whitneyi, Gabb (xxxvi, 81,89). Trias; Nevada, California. 
