56 CLASS III. GASTEROPODA. ORDER V. NUCLEOBRANCHIATA. 
passes through a wide degree of modification. The ventral disc, for 
example, which is so prominent a feature in the rest of the class, is here 
represented by a gelatinous natatory fin; and the animal, scarcely ever 
at rest, is made to swim longitudinally on the surface of the water with 
its shell downwards. 
The following genus is the only one of the family of the Nucleobran- 
chiate Gasteropoda that can be said to be conchiferous : 
CARINARIA. 
CARINARIA, Lamarck. 
Testa conica, pileiformis, tenuissima, hyalina, lateribus subcompressa ; 
vertice valdé attenuato, in spiram incurvam plus minusve reflexo, 
dorso carina aut simplici, aut dentata, instructo ; apertura oblonga, 
amplissima, integra ; impressione musculari nulla. 
The genus Carinaria was instituted by Lamarck for the purpose of 
distinguishing a beautifully transparent glassy shell, which, in the abs- 
ence of its animal inhabitant, had been regarded by Linnzus as a spe- 
cies of Patella. Gmelin was as ignorant as his predecessor of the true 
nature and purposes of this shell ; but upon considering its very apparent 
want of affinity with any of those of the Patelle, he was induced to 
remove it, in the 13th edition of the ‘Systema Nature,’ to a place 
amongst the Argonauts. Here, however, it was not destined to remain 
long, for upon the discovery of the soft parts, the Carinariz were found 
to be much larger animals than had been anticipated; the purpose of the 
shell, as we have already stated, being merely to cover a small dorsal 
nucleus containing the organs of respiration. 
The Carinarie, by their peculiar system of organization, have given rise 
to much speculation amongst naturalists as to the situation they should 
occupy in the system. Lamarck continued to place them next to the 
