ORDER III. 



^ntbalb($. 



I, SHELL OF ONE PART ONLY, AND HAVING A REGULAR 

 SPIRE. 



ARGONAUTA Paper Sailor. 



DESCRIPTION OF PLATE XVIII. 

 Dnr. l.—Fam. 1. Fig. 1. A. argo. Fam. 1. Fig. 2. A. tuberculata. 



Shell univalve, spiral, involute, membranaceous, one-celled. 



THE shells of this g-envis 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. 



