266 EMMERICH ON THE MORPHOLOGY, CLASSIFICATION 
the caudal shield, and to form a new genus of it, which however 
would be solely distinguished by the number of the thoracic 
rings. The difference of the two genera would certainly not be 
less than that existing between the two genera Porcellio and 
Oniscus, of which the one possesses 7-jointed external palpi, 
the other 8-jointed ones. Such a splitting up of closely allied 
Species appears to me however purely artificial, and therefore 
unjustifiable. I consider even the uniting of all 8-, 9- and 
10-jointed Trilobites into one genus as more natural than such 
a division, the more so as, with all the difference,of the ex- 
treme forms, we find them linked together by the interme- 
diate forms of 4. palpebrosus and A. leviceps. All these 8-, 
9- and 10-jointed Trilobites are unquestionably more nearly re- 
lated to one another than the genera Nuttainia, Portl., Brontes, 
Gerastos, Goldf., and Phillipsia Dalmani, to the 10-jointed Tri- 
lobites (I//enus and Bumastus), although they all possess the 
same number of joints, namely 10. The Trilobites just enume- 
rated prove most evidently, that Trilobites which are widely dif- 
Serent from one another may nevertheless correspond in the num- 
ber of thoracic rings. ITlienus centrotus, on the other hand, as 
also the 10-jointed -4saphus Dalmani, which correspond in all 
the other generical characteristics with the 9-jointed Phillipsie 
of Portlock, make it appear suitable to unite, by way of exception, 
Trilobites with an unequal number of joints into one genus. 
The differences in the structure of the rings of the thorax are 
not very numerous; I will only remark that we are able to draw 
inferences with perfect certainty from their structure as to the 
existence of the power of contraction. It is by no means un- 
common, that Trilobites which have possessed the power of 
doubling themselves up into a perfect ball, are found in an ex- 
tended position, and if this was to be considered as a proof of 
the want of contractibility, it would frequently lead us into se- 
rious errors. It is therefore not unimportant to possess a cha- 
racter which may guide us to a safe decision. Such a character 
exists. The margins of the fins, for instance, have so fine an edge 
in all the contractile species, that this sharpened anterior margin 
of the posterior joint is enabled to insert itself beneath the pos- 
terior margin of the preceding joint; the front fin, when in the 
act of contraction, inserted itself beneath the posterior margin 
of the cephalic shield; the anterior margin of the caudal shield, 
on the other hand, under the last joint of the thorax. This 
