264 EMMERICH ON THE MORPHOLOGY, CLASSIFICATION 
vided by lateral incisions into lobes, of which there are then 
three on each side *. 
Behind the head is situated the articulated thorax, in which 
the differences are limited to the number and structure of. the 
rings. The number of rings is definite and constant in each 
species, according to a law deduced by Wahlenberg from nume- 
rous observations ; Dalman found the same in all the species in- 
vestigated by him, and Professor Quenstedt has likewise con- 
firmed it more recently. It is therefore no longer a contestable 
fact, and we should rather ascribe the uncertainties of number, 
which we still occasionally meet with in books, not to the nature 
of the animal, but rather principally, if not exclusively, to the 
imperfect condition of the fossils, and to the consequent difficulty 
in counting the joints. The posterior margin of the cephalic 
shield, which is formed by the furrow of the neck, as also the 
similarly shaped anterior margin of the tail, have frequently 
given rise to a false calculation of the rings of the thorax. 
Their number is decidedly fixed with respect to the individual 
species, but whether it is so also with regard to the genera is 
another question. In many cases there can be no doubt of it, 
in favour of which assertion we may adduce the partially natu- 
ral groups which Quenstedt obtained by placing together the 
species with equal numbers of joints. Six thoracic rings, the 
smallest known number, occur in Ampyxr and Cryptolithus ; 
seven only in Ogygia; thirteen is a characteristic number for 
Calymene and for Homalonotus, so nearly related to it; eleven 
for Phacops. It is different however with respect to the 8-, 
9- or 10-jointed Trilobites, which Dalman enumerates in his 
divisions of Asaphi genuini ecaudati and mutici, Nileus and Ji- 
lenus, to which Murchison’s Bumastus is most nearly related. 
Are we now to unite all these Trilobites in one large genus, 
Asaphus (Cryptonymus, Kichw.), as I have done, subsequently 
dividing the latter into subgenera, or should they be divided ac- 
cording to the number of joints, leaving other reasons of divi- 
sion out of consideration? The latter view is the one generally 
entertained ; indeed some persons have gone further, and have 
* These sections, the number of which, including the neck furrow, never 
exceeds four, may lead us to infer the real number of rings, of which we may 
conceive the cephalic shield to be formed. The posterior margin of the ce- 
phalic shield has entirely the shape of a common ring of the thorax; and in 
some few the two hindermost sections of the frontal protuberance likewise 
form a second ring. 
