£ 1 Dr. Grav’s Obfervations on the Clafs of 
clafs (which fome naturalifts have cavilled at) becomes much 
lefs objectionable ; there being few genera, in the two orders of 
which it is now prefumed to confift, which do not contain 
animals to which the term amphibious may, with fome pro* 
priety, be given ; whereas, in the order of Nantes, not one 
fpecies occurs which has the fmalleft claim to that title. With 
refpeCt to the other error I noticed (viz. that of fuppofing the 
hearts of the Amphibia to be fingle) it would be ealy to fhevv 
that it was not an uncommon one, at the time Linnteus 
formed his fyftem. And indeed he appears to have been led 
into it, by following an author whom he probably fuppofed 
of too great fame not to be fafely relied on. At leaft, in de- 
fence of his opinion, he quotes the following words of Boer- 
haave. <c In omnibus animalibus in quibus fanguis non calet , ven- 
“ triculus cordis ef unicusD Whether the hearts of all the dif- 
ferent genera, of which the clafs is compofed, have yet been accu- 
rately examined ; and whether an exaCt fimilarity of ftruCture is 
found throughout the clafs ; are queftions 1 do not mean, at 
prefect, to examine. It is fufficient for my purpofe to obferve, 
that the hearts of moft of the Amphibia are now well known 
to be double, with an immediate communication between the 
‘two cavities ; which ftruCture feenis peculiarly adapted to that 
change of element, which (as I before obferved) many of them 
can, for a time, fnpport ; and thereby furnifhes another argu- 
meet in favour of the name Linn a: us has given to the clafs. 
To confider the ftruCture of the heart, however, is not 
abfoluteiy neceflary in forming the characters of the clafs i 
the animals of wHtch it confifts being fufficient ly diftinguifhed 
from all others, by having cold red blood, and breathing by 
means of lungs. Thefe two characters render the clafs per- 
fectly difti nCt from the reft 5 the two fuperior ones, viz. 
4 Mammalia 
