AND DISTRIBUTION OF TRILOBITES. 265 
even ascribed the rank of genera to the sections based upon the 
more or less deep longitudinal and transversal furrows. It is my 
conviction that this must lead to an objectionable subdivision. 
Thus we distinguish now true Asaphi (Cryptonymus, Kichw., a 
part of them at least), Isotelus, Symphysurus, Nileus, Illenus, 
and Bumastus, If the 9-jointed Iilenus centrotus possessed one 
joint more, it would remove all doubts at once; the 8-jointed 
species of Asaphus would in that case unquestionably form with 
Tsotelus and Nileus a distinct genus in the more narrow sense of 
the word, and the 10-jointed J//enus and Bumastus another ; 
to the former we might leave the name of Asaphus assigned to 
it by Brongniart, to the latter the name of J//enus might be 
given, although many 8-jointed species approach nearer to 
the 10-jointed in habit than to others of the division; for in- 
stance, Jilenus, and Asaphus palpebrosus, which M. von Buch on 
that account likewise considered as belonging to I/lenus (Con- 
tributions to the Geology of Russia, p. 51). In this case we 
should have to regard the species of the two genera as two par- 
allel series, made up of corresponding forms, as we see similar 
parallel series of genera exhibited in whole families ; for instance, 
the series of the Nautilaceous genera (Orthoceratites, Cyrtoce- 
ratites, Nautilus) are repeated in the family of the Ammonites 
(Baculites, Hamites, Ammonites). 'The species of Nileus would 
in that case be analogous to Bumastus, Asaphus palpebrosus, &c. 
to the Illeni; Asaphus extenuatus, however, would be analo- 
gous to Illenus centrotus; only Asaphus expansus and the 
Griffithides would not exactly correspond. But the often-men- 
tioned I//enus has not ten joints, but only nine, according to 
the statement of the accurate and careful Dalman, both in the 
diagnosis of the species and section. Nevertheless, we might 
still be in doubt about it, notwithstanding the accuracy of 
Dalman, who would certainly have put a mark of interroga- 
tion in any doubtful case, if another observer equally credible, 
Capt. Portlock, had not likewise represented two Irish speci- 
mens of the same species with nine thoracic rings. If therefore 
we were to unite only species possessing an equal number of 
thoracic rings into one genus, there would be no alternative but 
to separate this last-named trilobite, for instance, from its next 
of kin “ I/lenus crassicauda,” with which it completely corre- 
sponds in point of habit, form of forehead, course of the facial 
line, position of eyes, structure of the rings of the thorax and of 
