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A variety of authors have made the hiftory of 
Shells their ftudy; and, with indefatigable pains, 
they have fyftematized them into different claffes, 
families, genera, and fpecies. Different diftinc- 
tions have been adopted by different concholo- 
gifts; but the general divifions of Ariftotle have 
always been retained by the judicious, whatever 
alterations they have chofe to make in the fubor- 
dinate parts. It is impofTible to fpecify all the 
fyftems of ingenious men; but that of Da Cofta 
being at once one of the mofl recent and moft fa- 
fisfaftory, we fhall fubjoin a general view of his 
method, and leave the curious in Shells to per- 
feft their knowledge by confulting the original. 
This accurate conchologift begins with afcer- 
taining fome effential chara6lers by which Shells 
may be divided into families or claffeSj genera and 
fpecies. Thefe charaflers muft be formed from 
the principal parts of the Shell, the variations of 
which in fize, fnape, or fituation, will enable us to 
eftablifli the feveral fubdivifions. Thus, uni- 
valves have the five fubfequent eflential charaflers 
for the claffes or families; fimple, or not turbi- 
nated, with a fingle continued cavity, turbinated 
and chambered, or with many cavities, the total 
fhape, and the aperture or mouth of the Shell. 
The fubordinate characters for the genera and 
fpecies of univalves are alfo five; the number of 
fpires or wreaths, operculated or not operculated, 
the nature of thie flielly fubflance, whether opake, 
corneous, or pearlaceous, the epidermis and the 
head, and extremity or tip. 
Da Coila adheres to the Ariftotelian diftribu- 
tion ; and all thofc bodies which coincide in one 
effential charadler, he refers to the fame clafs; 
whilft the affinities or differences of thefe bodies 
to each other, in fuch parts as are not deemed 
principal, conftitute the fubordinate genera and 
fpecies. Accordingly, he fixes on the aperture 
or mouth of the Shell as the diftinguiftiing cha- 
raCleriftic of the turbinated univalve; on the 
hinges for the bivalves; and the number of valves 
for the multivalves. The fimple figure, the cham- 
bered ftru6lure, or the latent convolutions of the 
revolved Shells, which are thofe univalves uncha- 
rafterized by the mouth, as the limpets, ammo- 
nia, and cowries, are made the diftinguifliing cha- 
raflers of thefe families. The figure or fhape, the 
turban or clavicle, the colour, confiftencc, and 
ftreaks on the Shell, are confidered as the fecond- 
ary charafters of genera or fpecies. 
Univalves of the firft clafs comprehend four 
general fubdivifions or orders. The fimple uni- 
valves, or thofe which are not turbinated, and 
very flightly fpiral. This part contains four fa- 
milies; the limpets or patelte; the aures marinas, 
which are flightly fpiral; the vermiculi, or worm 
Shells ; and the dentalia. 
The firft family of the limpets is again fubdi- 
vided into three genera; the whole or entire, with- 
out a perforation at the top; the chambered; and 
the pierced, or perforated, with a hole in the top 
quite through the Shell. The firfi: genus is very 
numerous; the fecond has alfo many fpecies; but 
the third has few. Europe affords but a fmall 
number of fpecies; the fineft and largeft are na- 
tives of fhe Eall Indies; America has many of the 
chambered and fmaller kinds ; and fome large and 
beautiful limpets have recently been imported 
from the Straits of Magellan and the South 
Seas. Limpets in a foffile ftate are by no means 
common. 
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The fecond family is the haliotes, aures ma- 
rinas, or fea-ears, called alio ear fnells. There 
are very few fpecies of this family; and of thofe 
which have been propofed as diftinfl fpecies by 
feveral conchologlffs, fome are certainly no more 
than varieties. Da Coffa fays he never knew one 
inftance of an haliotis being found foffile. 
The third family is the vermiculi, or worm 
Shells. Thefe are fubdivided into vermiculi, or 
tubular worm Shells, which have no fixed or de- 
terminate form ; and penecilli, or thofe worm 
Shells which in the whole, or any efpecial or par- 
ticular part, have a determinate regular fhape or 
ftrufture. There are but few fpecies of either kind. 
The fourth family is the dentalia. Thefe are 
fimple tubular Shells, of a regular, determinate, 
curved, conical fhape, open at both extremities. 
This family contains very few fpecies; nor are 
there many foffile ones difcovercd recent. 
The fecond fubdivifion of univalves confifls of 
fuch as are concamerated or chambered, havins: 
many regular and nearly equidiftant cells or cham- 
bers; and a pipe, or fiphunculus, which opens 
into and communicates from chamber to cham- 
ber. This fecond part, which conflitutes the fifth 
family, contains fix genera; one genus of which, 
the orthoceratites, is of a fimple figure: four ge- 
nera, as the lituitJE, or crofier, turbines polytha- 
lami, ammonia, and ammonoidje, are all turbi- 
nated ; and the other genus, or nautilus, is re- 
volved. There are only two of thefe fix genera 
that are known recent, the lituitse and nautilus;, 
and therefore the others belong to the foffile 
kingdom. The lituit^ exaftly refemble a bi- 
ftiop's crofier in ffiape, having a long cylindric 
flem, one end of which turns in a fpiral manner j 
but the fpires are few, feparated, and receding 
from each other. This genus was firft difcovered 
by Breynius, and is feldom found in a foffile fbate. 
The other reeent genus, or nautili, are revolved 
Shells, or thofe whole fpires never appear exter- 
nally, but are very latent within the fabric of the 
Shell. They are of a chambered flrudlure, the 
partitions of the cells or chambers being concavo 
convex roundifh plates. The paper nautilus, fays 
Da Cofta, though claffed by moft authors as a 
nautilus, is a diftindt genus from this, being de- 
void of a chambered ftrudture, which is the effen- 
tial charafter. The fpecies of nautili are few. 
Conchologifts make two fpecies of the Indian or 
pearly kind; the umbilicated and non-umbili- 
cated. The inclofed animal is faid to inhabit 
only the uppermofl or open chamber, which is 
much larger than the others: the reft remain 
empty, except that the pipe or fiphunculus, which 
communicates from chamber to chamber, is filled 
with an appendage or tail of the animal, refemb- 
ling a gut or ftring. This fiphunculus is a di- 
lateable tube, under the direftion of the animal : 
when depreffed, like the fwimming bladder of a 
fiffi, it renders the nautilus buoyant; when it is 
contracted, the fifh and Shell fink, and to fuch a 
depth as the prefenc occafions of the creature re- 
quire. 
The third fubdivifion or order of univalves 
comprehends revolved Shells, or thofe whofe 
fpires are latent within the body; and are never 
externally vifible, being entirely deftitute of a cla- 
vicle or turban. This third part, which is the 
fixth family, contains three genera; the nuces, or 
bulljE; the femi-porcellan£ei and cypreas, or por- 
cellanje. 
The 
