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. lUaenus 

 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, strongl}^ 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, Illaenus americanus Billings is an American species 

 which is very similar to /. 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. 



Deucalion Shtsheglov, 1827. 



Sur les Trilobites en general et en particulier sur ceux de Zarskoe- 

 Selo. Journ. fur neue Endeckungen in der Phys. Chem. Natur. und 

 Technologic St. Petersburg, 1827, no. 1, 2, p. 234, pi. 7, f. 9 a-c. 



I have not seen this paper, but judge from what Holm says that 

 Deucalion is a svnonvm of Illaenus. The genus was founded on a new 

 species, D. hrongniarti, 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 



