RAYMOND: NEW AND OLD SILURIAN TRILOBITES. 5 
Dalman divided Asaphus into four sections, the third of which he 
designated as Illaenus, using the name in a subgeneric sense. Illaenus 
he divided into two divisions. Divisio 1, Cornigeri, contained the 
single species Asaphus (Illaenus) centrotus, which Burmeister made the 
type of Dysplanus in 1843. Divisio 2, Mutici, contained Asaphus 
(Illaenus) crassicauda and A. (Illaenus) laticauda. * The first of these 
has by general consent been made the type of Illaenus. Holm has, 
in several papers, redescribed the typical species, so that its charac- 
teristics are well known. Beside the characters of the family, the 
species shows a short and wide, strongly convex and curved cephalon 
and pygidium, both without concave borders, large prominent eyes 
which are situated far back, deep but short glabellar furrows, short 
but rather wide free cheeks without spines on the genal angles. The 
thorax has a narrow axial lobe and ten segments. The pygidium has a 
short but prominent axial lobe, and is wider than long. As Clarke 
has pointed out, I/laenus americanus Billings is an American species 
which is very similar to I. crassicauda, and it seems that only such 
species as conform to the kind of structure exhibited by the type 
should be admitted to the restricted genus Illaenus. 
Dervcation Shtsheglov, 1827. 
Sur les Trilobites en général et en particulier sur ceux de Zarskoé- 
Selo. Journ. fiir neue Endeckungen in der Phys. Chem. Natur. und 
Technologie St. Petersburg, 1827, no. 1, 2, p. 234, pl. 7, f. 9 a-c. 
I have not seen this paper, but judge from what Holm says that 
Deucalion is a synonym of Illaenus. The genus was founded on a new 
species, D. brongniarti, which Holm was unable to recognize. 
Bumastus Murchison, 1839. 
Type, BuMASTUS BARRIENSIS Murchison (Partim). 
Silurian system, 1839, p. 656 (non figs.). 
The particular features of this genus upon which Murchison himself 
laid most stress were the absence of dorsal furrows, and the presence 
of ten segments in the thorax. The general usage, however, has been 
that of referring all illaenids having the axial lobe of the thorax very 
