374 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



the external, and much wider; the second lateral is usually entire, although 

 occasionally slightly denticulated. There is no auxiliary series. The internal 

 septa consist of a short bifid antisiphonal lobe, and a short entire lateral, just 

 inside the umbilical suture. These septa are very Hke those of Xenaspis 

 Waagen, except in the difference in the sizes of the lobes. This resemblance 

 becomes more important when it is known that M. aplanatuni has a body- 

 chamber at least three-quarters of a revolution in length. 



The writer regards Xenaspis and Gyronites as being very 

 closely related, and it is by no means impossible that 31. 

 aplanatuni may eventually be shown to belong to the former 

 genus. 



M. aplanatum White resembles Gyronites frequens 

 Waagen, but differs from the Asiatic species in lacking the 

 auxiliary lobe. It also agrees in septa with G. nangcensis 

 Waagen, but is slightly more involute, and has shorter lobes. 



The relative dimensions of M. aplanatiun are as follows: 



Diameter i.oo 



Height of last whorl 0.37 



Height of last whorl from the preceding 0.27 



Width of last whorl o. 20 



Width of umbilicus 0.37 



Involution o.io 



In small specimens the whorl is more robust, broader in 

 proportion to its height, and more deeply embracing, 

 although the form is always very evolute, even in early stages 

 of growth. The young whorls are rounded, the angular 

 venter being a character of adolescence. The adolescent 

 shell agrees in all respects with Lecanites Mojsisovics, and 

 this species gives a transition from that genus to Meekoceras. 

 On this account the writer prefers to recognize Gyronites as 

 a subgenus, or transitional group. 



Hyatt^ took M. afJanatiim as the type of a new but unde- 

 scribed genus, Wyomingites, but this statement was intended 

 to refer to the form named by Waagen Gyronites -whiteanus, 

 and not to the typical Af. aplanatuni. It was unknown to 

 Hyatt that Waagen had already renamed this form. 



Horizon and locality. Lower Trias, Meekoceras beds, 

 southeastern Idaho, Aspen Mountains, about five miles west 

 of John Gray's Lake, also about fifteen miles a little west 



^Cephalopoda 1900, p. 556. 



