SPHENODISCID®. 63 
The first specimen is the only one with perfect young that I have seen, 
and is, therefore, a very instructive example (Pl. III, figs. 9-12). In 
ananepionic stage the innermost volution next to the large protoconch is 
rounded and followed by a volution still in the nepionic stage. This 
acquires an elevated venter and becomes more compressed and helmet 
shaped, but has nowhere a flattened or concave venter. The sutures were 
simpler than those of later age. The sutures on both sides had five divided 
saddles. he first lateral saddles were bifid on both sides, the second just 
beginning to be quadrifid, the third and fourth symmetrically trifid, and 
the fifth only hifid on the right; whereas on the left side the second to the 
fifth, like the first, were all bifid. The volutions in section were similar in 
outline to the adult, having the same highly involute aud almost pear- 
shaped section and acute venters. Neither of these specimens showed the 
living chamber. 
One specimen (Loc. 582, U. 8. Geol. Survey, Pl. V, figs. 1, 2), when 
compared with more typical forms, shows, in what is probably the mete- 
phebic substage, the outer fold-like costee as in the gerontic stage of others. 
The diameter of the cast, without the shell, is about 118 mm., partly esti- 
mated. The last of the outer volution measures 60.5 mm., the umbilicus 
8 mm., and the opposite volution from line of involution to venter measures 
about 49.5 mm. 
The acuteness of the venter decreases on last part of this volution, 
but does not become blunted and rounded as it does at the corresponding 
size in the typical form. 
The sutures of the last five septa show gradual approximation, and in 
the closer approach of the last two there are indications that the specimen 
was beginning its gerontic stage. The living chamber was broken away, 
but the marks of the umbilical parts showed that this had extended at least 
one-fourth of a volution farther on the sides. The inner line of tubercles 
was becoming wider apart, and together with the venter and sutures, also 
indicated that the gerontic stage was begun or was about to begin, and had 
been perhaps nearly completed in the now absent living chamber. The 
sutures, it will be understood, always represent a later stage than the parts 
of the shell on the inner surface of which they are found. The sutures 
have short phylliform saddles and broad lobes, and are well separated from 
each other and approximate only near the umbilical shoulders. They were 
