UNIVALVES. 



PLATE XLII. 



Genus. ARGONAUTA. 



Character. Shell semiorbicular, flattened centrally, convolved, shaped like the 

 Cornu Ammonis, environed at the outer edge with a membranaceous ligament, nar- 

 rower than the rest of the shell, and doubly tuberculated; the inner edge rounded. 



Species. 



No. I. Argonauta vitrea. Shell white, transparent, elongated, divided trans- 

 versely by lineal divisions ; the outer edge membranaceous and articulated. 

 This curious shell was formerly noticed by the celebrated M. Lyanet of 

 Amsterdam, who first described it. Mr. Jennings of Chelsea has a speci- 

 men of it : there is also another in the British Museum, from which this was 

 delineated ; it is esteemed to be very rare, and to be a native of some of the 

 southern latitudes. Nature seems to have sported in a wanton mood when 

 forming it, so differently from its congeners : in its general shape it may be 

 compared without impropriety to a Cornucopia. This shell has been erro- 

 neously placed by Humphreys amongst the Patellae genus, to which it has 

 no relationship or resemblance, as the Patellae are always without the liga- 

 ment. 



No. 2. Argonauta rotunda. Shell white, deorbated, transparent, slightly co- 

 loured with brown in the centre ; the outer edge tuberculous. A native of 

 the Cape of Good Hope. 



No. 3. Argonauta striata. Shell white, rounded, channelled. A native of the 

 East Indies. From a specimen in the Author's Museum. 



No. 4. Argonauta grandiformis. Shell large, white, very much flattened, 

 painted brown in the centre, outer ridge tuberculous. A native of the Cape 

 of Good Hope. Delineated from a shell in the Museum of Mr. Bullock. 

 The mouth appears as if broken off, which is a deceptio visus. 



REMARKS. 



The genus Argonauta possesses little beauty in its colours, but a symmetry exists in its form, 

 which is highly pleasing to the eye. It strongly resembles the well-known fossil shells called 

 by the name of Cornu Ammonis, and it is not improbable that it is of the very same genus 

 and character. It is distinguished from the Nautilus, or chambered shell, by being simple 

 in its opening, and having no internal chambers or divisions. Its singular form sets it apart 

 and distinct from all other known species, and its apex or nucleus is always centrally regular 

 in its involution. The Argonaut can raise itself in the water, and by expanding a small sail 

 and eight little oars, skim agreeably along the surface of the main in fine weather, but upon 

 any alarm of danger it dives immediately to the bottom. 



