268 EMMERICH ON THE MORPHOLOGY, CLASSIFICATION 
depth of the transversal furrows, which are entirely wanting in 
Ilienus, but, above all, in the greater or slighter perfection of 
the joint between the articulations of the axis, and the free fins 
belonging to them, which joint disappears altogether in Bumas- 
tus and Nileus. 
I shall now conclude with a few observations on the caudal- 
shield. 1 have already stated that it consists of rings, which 
are only distinguished from those of the thorax by their being 
coherent with one another. ‘This cohesion differs considerably 
in degree; the most various intermediate stages exist between 
the caudal shield of Ni/eus, in which neither axis nor fins, and 
still less their articulation, is perceptible ; and Amphion, in which 
the coherent rings of the tail have so great a resemblance to the 
posterior rings of the thorax, that they can scarcely be distin- 
guished from them. This similarity renders the counting of the 
individual thoracic rings exceedingly difficult in some cases, and 
the erroneous statements in this respect may frequently be traced 
to this circumstance. The number of joints of the tail is cer- 
tainly equally constant for every species, as it is with respect to 
those of the thorax; their determination, however, is far more 
difficult than in the latter. The observations hitherto made do 
not yet sufficiently indicate what proportion the numbers of the 
rings of the tail and thorax bear to one another in the different 
species ; I certainly ascertained from some former researches 
which I instituted on this point, that the number of rings of the 
tail and thorax, in some species at least, is equal to that of all 
the rings of Paradoxides Tessini; and this may be the case with 
a great part of the family. Some, on the other hand, have evi- 
dently a smaller, and others even a larger total number, and I 
must for the present leave it undecided, whether in the former 
case the joints are coherent, or whether in the latter they are a 
multiple of the number of joints of the already mentioned P. 
Tessini. This is a question which certainly deserves more accu- 
rate investigation. 
The preceding observations are not intended to form a com- 
plete natural history of the Trilobites ; a profound work on this 
subject may soon be expected from one of the most distinguished 
investigators of recent Articulata, Professor Burmeister. My 
remarks are only intended to justify those alterations which I 
have proposed in the systematic arrangement, and, if possible, 
to confirm them. 
