JVo.1.] WHITE ON JUEA-TEIAS FOSSILS OF IDAHO. 117 



nience of future reference, and the characterization of the strata from 

 which it comes, I have applied the above speciiic name. 



This shell, hke those of the three species of Meehoceras which have 

 akeady been described, is dejiressed-discoidal ; the siphonal side rounded 

 in the young and ai)parently a little angular in the adult, but the latter 

 feature is not plainly shown in the exam^^les ; umbilicus open, but com- 

 paratively narrow ; sides of the volutions flattened, those of the outer 

 one of adult shells almost flat, or only slightly convex except near the 

 outer and inner sides ; the latter abruptly shouldered ; involution very 

 great in the young, but so lessening with growth that the outer portion 

 of the living-chamber of adult shells does not embrace by more than one- 

 fifth of its own width that next within. The examples do not show the 

 character of the sutures satisfactorily, but the lobes and cells are both 

 seen to be constricted about the middle, the ends of the former oblique, 

 but digitate, as in MeehoceraSy and the latter plain, as in that genus and 

 Ceratites. Surface conspicuously marked by numerous longitudinal, 

 abruptly raised lines, which enlarge into ridges in the adult, and which 

 are separated by spaces a little wider than themselves ; being also appa- 

 rently continuous from the young state to the fully adult, so that the 

 ridges and spaces, especially the latter, increase in width with the growth 

 of the shell. 



This species is much larger than either of the other species of Cepha- 

 lopods which are described in this paper ; the extreme diameter of the 

 coil at fully adult size being not less than 180 to 200 millimeters ; and 

 the vertical diameter of the outer portion of the living-chamber about 

 80 or 85 millimeters. 



Position and. locality. — Jura-Trias strata, member D of the foregoing 

 section 5 at locahty No. 1, Southeastern Idaho. Collected by Dr. A. C. 

 Peale. 



Arcestes ? *? — Three or four young examples only of this species 



were obtained by Dr. Peale from member D of the section at locality 

 No. 1, shown on a previous page, all of which are too imperfect for spe- 

 cific determination. They are, however, so very like A. gahU Meek, Vol. 

 lY, Geol. Expl. 40th Parallel, p. 121, pi. x, figs. 6, 6% and O'', as to leave 

 upon the mind a strong impression of their identity. 



Arcestes"? ? — Associated with the foregoing, a single speci- 

 men of another species, also very young, was obtained, which differs from 

 the last in'ha^dng a much more open umbilicus, and in the proportion- 

 ally greater transverse diameter of the volutions. 



