The Genus Xanno, Clarke. — Hyatt. 11 
veloped and it is obvious that the swollen end is sometimes 
made up of the funnels of from four to six septa and that the 
tip of the siphon is a coecal cone like that of other forms of 
nautiloids, differing principally in size. 
In good si^ecimens the extreme tips of these siphons bear 
marks of having been perforated by a hole thatcoramunicated 
with the apex of the shell layers, which seemed to have been 
plugged up as the siphon became filled with organic deposits. 
The evidence of this consists in the lighter or darker color of 
a central spot that appears on the apex of isolated siphons 
and is often much more decisive than the external marks on 
any specimen of Nanno. This spot is sometimes circular but 
may also be elliptical or irregular, and it differs usuall}"- in 
color from the outer surrounding surface and is continuous 
internally with the endosiphon. 
The remarkable transitional form described by Clarke as 
Orthoceras with a protoconch makes the conclusion previ- 
ously drawn from the existence of the cicatrix on the apices 
of JSfautilus, Orthoceras., and Piloceras, much more reliable 
than it was previously. This discovery confirms the opinion 
that the division of the early stages into protoconch and 
conch is true probably of all nautiloids and the cicatrix upon 
the apex of the true conch is probably in all cases to be ac- 
cepted as due to the removal of a protoconch. 
This being true, the very close similarity of the younger 
stages of Nanno, Sannujnites, Vaglnoceras and Endoceras to 
Piloceras, makes it highly probable that the apex of the shell 
will be found to have an external cicatrix in the former, 
although it may not be such a deep pit as that already dem- 
onstrated in Piloceras by Ford and confirmed by my own ob- 
servations upon the species in this country. 
EXPLANATION OF PLATE L 
Nanno aiilema Clarke. 
Fk;. \. Fragmeiil of tho end of the siphon of aspocimon in the ephe- 
bic stage, enlarged two diameters. This sliows the endosiphon in the 
first endocone, filled by the red ferruginous matrix and closed internally 
by organic deposits, aiso the loose cliaracter of the remaining internal 
deposits, the hollow formed in the center by the absence of these depos- 
its and a part of the narrow t\ibular end of the sheath above. Enlarged 
-|- 2 diameters. Locality, Chatfield, Minn. 
Fi(i. 2. The entire siphon of a younger specimen in the ananeanic 
