AND DISTRIBUTION OF TRILOBITES. 267 
sharpening of the margin by a surface which becomes broader 
and broader from the interior externally, admits of our forming 
an opinion as to the existence of the property of contractility, 
even in straightened individuals. The contractile power cer- 
tainly depended in a still higher degree on the structure of the 
joint between the articulations of the axis, but since all Trilo- 
bites were able to bend and extend the whole body, the differ- 
ence in the formation of the joint can only be a relative one, and 
therefore one that is difficult to handle; this is not the case with 
the fins. Brongniart, and several later authors, have laid great 
stress on the absence or presence of the power of contraction, 
and certainly not without reason. It is most characteristic for 
several genera; but what I endeavoured to prove with regard to 
the number of individual joints also holds good in the present 
instance, viz. this character is likewise not of equal value 
throughout the whole family. As an instance of this, I will 
mention Odontopleura, a genus which I have recently established, 
differing from all other Trilobites in habit as well as in several 
characters. Of the three species with which I have hitherto be- 
come acquainted, one, O. bispinosa, was entirely deprived of con- 
tractile power, whilst a second, O. crenata, could double itself up 
completely; the third however O. mutica could only press cephalic 
and caudal shield against each other, and double itself up imper- 
fectly. These three species therefore exhibit every degree of con- 
tractility, and yet they undoubtedly belong (as will be seen from 
the subsequent descriptions) to one genus, and are not mere 
analogous forms of different genera. Even in the genus Phacops 
we find species with and without the power of contraction. The 
genera Calymene, Brongn. emend., Homalonoius, Asaphus, Brongn. 
emend., Illenus Dalmani, on the other hand, only contain con- 
tractile species with fins that are sharpened at the anterior margin 
and rounded at the extremity ; Ogygia (the 7-jointed Trilobites 
of Quenstedt), the Paradoxides, Cono- and Ellipso-cephalus, Nut- 
tainia, and Brontes, contain only species that do not possess the 
power of contraction, with fins that are not at all, or hardly, 
sharpened at the anterior margin. The apices of the fins are 
drawn out into long spines in some few species of different ge- 
nera (Phacops arachnoideus and Odontopleura bispinosa), a pe- 
culiarity which has been proved to be of secondary importance, 
since Odontopleure have been found without spines. In addition 
to this, the rings of the thorax also exhibit differences in the 
