Geol.— Vol. I.] SMITH— COMPARATIVE STRATIGRAPHY. 373 



Meekoceras, and that the rest of the species assigned to it 

 by Waagen would fall partly under Ophiceras and partly 

 under Danubites. Mojsisovics and Waagen formerly 

 classed the species of Gyronites under Xeriodiscus. Cer- 

 tainly the amount of involution should not be regarded as 

 sufficient excuse for generic separation, and only the de- 

 velopment stage can warrant such a separation. Now since 

 Meekoceras a-planatimi White, one of the original types of 

 Meekoceras in the broader sense, has all the characters of 

 Gyronites^ the writer prefers to retain this name as a sub- 

 generic title for the species of the group of M. aplanatiim. 

 Gyronites is found only in the Lower Trias of India, Si- 

 beria, Idaho and California, where it is represented by M. 

 aplanatum, and by several new species, as yet undescribed. 



Meekoceras (Gyronites) aplanatum White. 



Plate XLI, Figs. 4-6. 



1879. Meekoceras aplanatum, White, Bull. U. S. Geol. Sur. Terr. Vol. V, 



p. 112. 



1880. Meekoceras aplanatutn, White, An. Rept. U. S. Geol. Sur. Terr. 



Vol. XII, Part I, p. 112, pi. xxxi, figs, i, a, b, and d, (not fig. c, 

 which is Gyronites whiteaniis Waagen). 



1886. Xenodiscus aplatiatus, Mojsisovics, Arktische Triasfaunen, p. 75. 



1895. Xenaspis aplanatus, Waagen, Salt Range Fossils, Vol. II, Fossils 

 from the Ceratite Formation, p. 290. 



1900. Wyomitigites aptanattis, Hyatt, Cephalopoda, p. 556 (in Zittel-East- 

 man's Text-book of Palaeontology). 



1902. Ophiceras applattatuvi, F. Freeh, Lethsea Palaeozoica, Bd, II, Lief- 

 erung 4, p. 661, fig. e. 



Evolute, discoidal, laterally compressed ; wide shallow umbilicus. Whorls 

 increasing slowly in height, little embracing, outer whorl concealing but little 

 of the inner, and being indented to less than one-fourth of its height by it. 

 Breadth of whorl a little more than one-half of its height, and one-fifth of 

 the total diameter of the shell ; height of whorl a little more than one-third 

 of the diameter of the shell. Width of umbilicus equal to the height of the 

 whorl. Umbilical shoulders abruptly rounded, but not angular. Sides 

 gently convex, venter flattened, narrow, with subangular ventral shoulders. 



Surface ornamented with cross striae and folds, which may become quite 

 strong on the body-chamber. 



Septa ceratitic, saddles all rounded and entire, lobes partly serrated, partly 

 entire. The external lobe is divided by a siphonal saddle into two narrow 

 branches which are very slightly serrated ; these branches fall on the abdomi- 

 nal shoulder-angles. The first lateral lobe is distinctly serrated, longer than 



