ORDER III. 
nivaloyes, 
1. SHELL OF ONE PART ONLY, AND HAVING A REGULAR 
SPIRE. 
ARGONAUTA.— Paper Sartor. 
DESCRIPTION OF PLATE XVIII. 
Dry. Il.—Fam. 1. Fig. 1. A. argo. Fam. 1. Fig. 2. A. tuberculata. 
Shell univalve, spiral, involute, membranaceous, one-celled. 
THE shells of this genus are remarkable for their ex- 
cessive thinness and brittleness, and are perhaps surpass- 
ed by none in the delicacy and elegance of their struc- 
ture. The form resembles that of a scroll, with a 
very large aperture. The surface is ornamented with 
numerous canaliculated grooves, proceeding from the 
summit to the outer margin, which is in general bicari- 
nated; but there is one species, the A. vitrea, in which 
the margin is single, and the summit exteriorly curved. 
The species of the first division have the keel toothed, 
which is not the case with those of the second, and the 
distinction of the third division is, that the umbilicus is 
perforated. 
