m. 1.1 WHITE ON JUEA-TRIAS FOSSILS OF IDAHO. 113 



very considerable differences appear. The Iwvidorsatus being a llea^^ly 

 ribbed shell, even when quite large, with a rounded abdominal side ; 

 though here again I doubt if the sutures differ much. 



''The young shell of species "A" [3Ieel:oceras aplanatum] has rounded, 

 smooth whorls, which increase very slowly by growth, and are full half 

 an inch in diameter before the sides and abdomen (siphonal side) begin 

 to show the iiatuess which characterizes the adult. At about this size 

 also the whorls grow perceptibly broader in proportion on the sides, 

 and the amount of involution increases gradually until it covers about 

 one-third of the side, in a specimen about two and a half inches in diam- 

 eter ; the umbilical shoulder also changes at the same time, becoming 

 abrupt and subangular. 



"In some specimens there are indications of nodes on the sides, and 

 in some the young until a late period are distinctly ribbed, the ribs 

 being thick straight folds, reaching across the sides, but not up on to 

 the siphonal side (abdomen). 



"The age at which the serrations of the lobes appear could not be 

 seen, but great caution should be exercised in this group in describing 

 these characteristics, since they are liable to disappear with the removal 

 of the shell, on account of their shallowness. Thus a full-grown shell 

 may be readilj^ mistaken for a Goniatite 5 or the young, before the ser- 

 rations appear, for one of that genus." — (A. H.) 



Meekoceras musheachanus* White. — Shell compressed-discoidal, 

 having the peripheral side abruptly rounded ; umbilicus open, shallow, 

 about two-thirds as wide as the breadth of the outer volution in the 

 adult, and proportionally narrower in the young ; volutions flattened- 

 convex on their sides, the convexity merging gradually into that of the 

 peripheral side, but upon the umbilical side the volutions, especially 

 the outer one, are abruptly shouldered 5 volutions in the young distinctly 

 embracing, but the amount of involution increasing with the growth of 

 the shell, so that the outer one embraces nearly one half of the next 

 adjacent; siphonal cell of the septa, at or near adult size of the shell, 

 small, wedge-shaped, narrowing toward the front end, where it is slightly 

 notched ; middle and outer lateral cells of nearly equal size, larger than 

 any of the lobes, except in some cases the outer lateral one, somewhat 

 abruptly rounded in front ; inner lateral cell smaller than either of the 

 other lateral ones, its inner anterior border a little more abruptly 

 rounded than elsewhere; ventral and outer lateral lobes nearly equal in 

 size, or the former a little the larger than the latter ; their sides nearly 

 straight and their ends subtruncate, that of the ventral lobe bear- 

 ing six or seven narrow, prominent digitations, and that of the outer 

 lateral lobe eight or nine ; the inner lateral lobe about half as wide as 

 the outer lateral, but otherwise similar; auxiliary'- lobes and cells 

 between the inner lateral cell and the inner edge of the volution four 



* The spticilic name is given in liouor of Mr. J. E. Muahbach, assistant toxiogriii)lie.r, 

 wlio first discovered the species. 



Bull. V, 1 8 



