270 EMMERICH ON THE MORPHOLOGY, CLASSIFICATION 
extends almost as far as the furrow of the neck; the lateral lobes 
in this case disappear almost entirely. Thorax 9-, 10- or 11- 
jointed, joints furrowed, caudal shield with an crieniaes axis 
and ribbed sides. 
This family i is identical with Quenstedt’s group of 11-jointed 
Trilobites, in his treatise above alluded to. Although the law 
he has laid down in it, namely, that all Trilobites with a facetted 
horny membrane are 11-jointed, has not been confirmed, as, in 
addition to the 10-jointed Asaphus Dalmani, which was known 
at that time, other 9-jointed species have subsequently been dis- 
covered, having the same structure of eyes, yet it cannot be de- 
nied that this mode of classification is the first important step 
towards an improvement of Brongniart’s system. According to 
the course of the facial suture, the hitherto known species of } 
this family are divided into two groups. 
I. Phacops, m. (paxés, the lens, and a, the sight, eye). Facial 
suture extends to the external margin. Thorax 11-jointed. 
Professor Goldfuss has subdivided this genus established by 
me into three genera (Phacops, Asaphus, Acaste), but more 
according to habit than zoological characters. I certainly do 
not agree with this tripartite division, but nevertheless consider 
a further division of this genus as practicable. Brongniart ar- 
ranged a part of the species united here in the genus Calymene, . 
another part he placed among the Asaphi; all the allied species 
that were subsequently discovered were distributed according to 
the habit in a similar manner among the two genera. The dif- 
ferences in appearance of species, which caused for so long a time 
the separation of nearly related Trilobites from one another, and 
their union with others widely distinct, must necessarily be asso- 
ciated with geological differences. I believe I have discovered 
two essential distinctions between Calymene and Asaphi of our 
genus, which however require further confirmation; the one 
consists, in my opinion, in the presence or want of contrac- 
tility, the other in the number of rings which form the caudal 
shield ; in the latter, the Asaphi, they appear to amount to 
double the number possessed by the former. The difficulty, 
sometimes indeed the impossibility, of counting the hindermost 
rings of the tail, forms no trifling obstacle against the discovery 
of the numerical law here alluded to. The greatest number of 
rings of the tail known to me is in that of Asaphus Hausmanni, 
