SPHENODISCID %. Ue 
beginning with the fourth columns of saddles and lobes, both becoming 
simpler toward the umbilicus. This is parallel to the development of any 
row taken in the ontogeny, as explained in the general description. There 
are eleven lobes and twelve saddles, besides the arm of the ventral lobe on 
each side in the oldest part of this volution, and at least one, perhaps two, 
rows of lobes and one row of saddles less on the very earliest part of this 
volution when the shell was 26 mm. in diameter, and from. venter to line of 
involution about 9 mm. The saddles, when a little older than the suture 
figured, had the phylliform outlines of Meek’s figure and were plainly grow- 
ing to be more like his figure on pl. 34. Fold-like costee about 1 mm. 
apart occur, but these are elevated above the general surface only along the 
median surface of the sides. They are obsolete outwardly, and inwardly 
they are mere elevated limes. The involution almost completely covers the 
volution even at this stage, and the umbilical opening is only about 2 mm. 
in diameter. 
The venter is acute on the youngest part of this volution, and the 
shell is thin, the inner coat brilliantly nacreous; the keel is solid. This is 
probably the young of the same species figured by Meek on pl. 34, since 
some of the first lateral saddles already show the tendency to attenuation 
and the phylliform outlines of the marginal saddles common in that form. 
The siphonal saddle also has the same form as in that species, and the 
dependent marginal saddles on either side have the peculiar rounded aspect 
of the same parts as figured by him. The arms of the ventral lobe are 
long and directed laterally, and distinctly trifid. The whole diameter is 
42 mm., diameter at last septum is 30.5 mm., broadest volution from line 
of involution to venter 19.5, umbilicus 1.5 mm., and smaller part of same 
volution opposite 9.5 mm. 
Fragments of one or two specimens of the same species (PI. IX, figs. 
1-6) as the above, from Rock Creek, Wyoming (Coll. Yale Museum, 
No. 1697), show similar sutures and characters The age and size of 
this are about the same as of the one described above. There are seven 
narrow phylliform dorsal saddles in the zone of involution. The next 
to the youngest fragment of a volution was 4.5 mm. from venter to line 
of involution, with a transverse diameter of 2 mm. Even at this early 
neanic substage the form of the venter was like that of the adult and the 
involution almost completely concealed the next mner volution, which 
was paranepionic and quite distinct in form. The diameter of the youngest 
