m. 1.] WHITE ON JURA-TKIAS FOSSILS OF IDAHO. 115 



witliin ; siplional cell of tlie septa near the outer chamber of the largest 

 examples rather broad, shallow, and broadly rounded, occupying some- 

 times a little less and sometimes rather more than the whole of the 

 flattened portion of the siphonal side ; the outer and middle lateral cells 

 about equal in size and regularly rounded ; inner lateral cell shallower 

 than the others and broadly rounded, its inner border being defined by 

 a short abrupt curve backward ; the ventral and the outer and inner 

 lateral lobes all, except perhaps the outer lateral one, sm!aller than the 

 cells 5 the ventral one being smallest, wedge-shaped, and bearing two 

 slender digitatious, the inner lateral next in size, with four or five digi- 

 tations, and the outer lateral largest, with six or seven digitations ; the 

 auxihary lobes and cells occupying a space adjacent to the next inner 

 volution about as wide as that of the outer lateral lobe, and consti- 

 tuting a finely serrated suture, the lobes being minute and pointed, and 

 the cells a little larger and rounded at their ends ; the one adjacent 

 to the suture being a little larger than the others. Surface of young 

 examj)les nearly or quite plain, but in fully adult shells there is a ten- 

 dency to form nodes or ribs, the latter sometimes crossing the periphery ; 

 but they do not appear to assume that regularity which we find in typical 

 Ceratites. 



Diameter of the coil of the largest example in the collection, 100 mil- 

 limeters ; vertical diameter of the outer portion of the living-chamber, 

 45 millimeters ; transverse diameter of the same, 20 millimeters. 



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

 section ; at locality No. 1 and also at locality jSTo. 2, Southeastern Idaho. 

 Collected by Dr. A. C. Peale. 



The following are Professor Hyatt's remarks upon this species: 



" This species differs from "B " \Mee1coceras mushhacJianus] in about the 

 same way that "B" diifers from "A" [M. aplanatum] except in so far as 

 it approximates more closely to "A" in liaving a similar flattened abdo- 

 men. This flattened abdomen appears at a much earlier age than in the 

 less involute form, "A." In fact, before the shell reaches the diameter 

 of three-sixteenths of an inch not only is the abdomen flattened, but the 

 sides also ; and the increase by growth is so rapid that the sides of the 

 internal whorls, even before this period, are almost entirely hidden. 

 This is therefore similar to those forms among Ammonites^ wliicli I have 

 so often described as accelerated types, those which display in the earher 

 periods of growth and developement, in quick succession, characteristics 

 which come out in slower* succession in other species, Like many of 

 those forms also, a kind of premature degeneration appears, even before 

 the animal can be said to have reached its adult condition. Thus, at 

 the diameter of an inch and a half, or even less sometimes, the sides of 

 the whorls no longer increase by growth with the same rapidity as in 

 the young. The amount of involution consequently is not maintained 



* " This expression, of course, is relative ; applying not to the absolute amount of time 

 occupied in the gro-wth, but to the age at which the characteristics appear." 



