THE 
AMERICAN GEOLOGIST 
Vol. XVI. JULY, 1895. No. 1. 
REMARKS ON THE GENUS NANNO, CLARKE. 
By Alpheus Hyatt, Boston, Mass. 
(Plate I.) 
The discovery and description of this genus by Prof. J. M. 
Clarke has materially added to our knowledge of the structure 
and development of the siphon in the Endoceratida' and 
thrown a new light upon the aftinities of the forms of this 
grouj). The endosiphon* is absent in the maturer stages of 
jSTaiino and probably also in Varjiiioceras, and in Sanntonites 
it appears to be present only in a fragmentary form in some 
species. Prof. Clarke's original specimens in some cases dem- 
onstrate the existence of an endosiphon in the young of Xan- 
no and that it was in one stage an open tube. This gives a 
standard by which to judge of the aftinities of the genera in 
the Endoceratid(i: and shows clearl}^ that Endoceras is the 
radical from which the genera Sannioniies, JSfanno and Vaginn- 
cerns were derived. Prof. Clarke having generously al- 
lowed me to study his types, I have made careful observa- 
tions upon these interesting fossils. The specimens figured 
in his platef, figures 5-7, are in my opinion the young of fig- 
ure 1, as described b}'^ Clarke, and so are also figures 2 and 8. 
Figure 3 represents a full grown siphon of a specimen older 
obviously than figure 1. 
*I have used the term siphon in this paper Ui facilitate comparison 
and reference to Clarke's paper. I usually use the term si ph uncle for 
the calcareous tube and siphon for the fleshy tube. 
\"N<irmo, a new Ce|)ha!opodan type," .T. M. Clarke, Am. Geologist, 
vol. XIV, p. 205, pi. «, Oct., 1894. 
