112 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Yol.Y. 



been compared, though no exact observations could be made for want of 

 good specimens of the young of true Ceratitesy — (A. H.) 



Meekoceras aplanatum White. — Shell compressed-discoidal, hav- 

 ing the peripheral or siphonal side of the outer volution flattened, that 

 of the inner volutions being a little rounded, and although narrow, its 

 breadth is considerable as compared with the slight transverse diameter 

 of the volutions ; umbilicus open, shallow, its width in the adult being 

 about equal to that of the greatest vertical diameter of the outer volu- 

 tion, but it appears to have been i^roportionally wider in young exam- 

 ples ; volutions flattened-convex on their sides, but their inner edges 

 are abruptly rounded inward to meet the next volution ; all the volu- 

 tions slightly embracing, the inner ones apparently more slightly than 

 the outer. Siphonal cell of the septa, near the outer portion of the 

 largest example obtained, small 5 the outer, middle, and inner lateral 

 cells larger, regularly and plainly rounded and of about equal size ; the 

 outer lateral lobe of about the same size as the lateral cells, bearing at 

 its rounded extremity five or six small digitations of uniform size ; inner 

 lateral lobe smaller than the outer, bearing about four digitations like 

 those of the outer one ; ventral lobe very small, about equal in size with 

 the ventral or siphonal cell, and apparently simply notched at the end ; 

 no auxiliary lobes or cells apparent in the examples yet discovered. Sur- 

 face of the outer volution nearly plain, but the sides of the small inner 

 volutions marked by moderately distinct transverse ridges. 



Diameter of the coil of the largest example in the collection, 60 milli- 

 meters ; extreme vertical diameter of the outer volution, 22 millimeters j 

 tranverse diameter of the same, 12 millimeters. 



Position and locality. — Jura-Trias strata ; locality ISTo. 2 of the preced- 

 ing pages. Southeastern Idaho. Collected by Dr. A. C. Peale. 



Professor Hyatt makes the following remarks upon this species : 



" This species belongs to that group of this genus in which the shells 

 have but a slight amount of involution, and possess also the narrow and 

 numerous whorls which are the invariable accomiDaniments of this char- 

 acteristic in all the Ammonitoids and l^J^autiloids. The resemblance to 

 Amm. parous von Buch* is very close so far as the outlines of the sutures 

 are concerned, but the sides are too flat, the umbilical shoulder very 

 abrupt, and the siphonal side flattened. There is an equally close resem- 

 blance to Ceratites carbonarius Waagen,t but here again the siphonal 

 side is flatter and the involution greater, the last whorl at the same size 

 as in the largest one figured being broader on the side. The septal 

 sutures are, however, very similar. 



"The young, when of the same size, are very similar to the figure of 

 Goniatites IcEvidorsatus Gabb,| but when larger, as in Meek's figure, § 



* Ueber. Ceratiten. Abliand. Akad. d. Wissenscli. 1848, pi. 4. 

 t Memoirs Geol. Surv. India, Pal. vol. ix, art. 4, pi. 1. 

 X Pal. Geol. Surv. California, vol. 1, pi. 3. 

 § Geol. Expl. 40tli Parallel, vol. iv, pi. x. 



