182 



SILURIAN TRILOBITES. 



maintained, though under considerable variations, throughout the North and Central 

 European types and those of Canada and North America. The greatest variation is 

 found among the oldest forms, viz., those from the Arenig and Llandeilo rocks, where 

 the genus first appeared. If we look to our plates, we shall find that the ordinary 

 species of lUcenus have the furrows which mark out the glabella very short externally, 

 and the eye placed at a moderate distance or somewhat remotely from the glabella, and 

 behind the middle of the cheek. But in the oldest group of Silurian rocks known 

 to contain IllcBnus, the eyes are placed so far forward and outward, that in one form, 

 Edillanus (PI. XXVI, fig. 8), they seem at first to be absent altogether ; and in 

 Hydrolanus, a Canadian subgenus {Illeenus conifrons, Billings), the glabella-furrows are 

 so long as to reach the front, the glabella so gibbous as to overhang it, and the eyes 

 are extremely forward. These two subgenera lead directly to Illcsnopsis (PI. XX, fig. 1), 

 which is from the Arenig rocks (it has been quoted from May Hill Sandstone !). 



If we divide Illsenus into the undermentioned sections, following the lead of 

 previous authors, it is not because we think the divisions quite natural, but because 

 they are convenient. Some of the subgenera will be hereafter elevated to generic rank 

 when we know more of the lobes of the head, not often visible even in internal casts. 



§ 1. — Trilobed: labrum elongate. 



1. Odillanus, Salter, 1866. Eight rings; a well-defined glabella, and the upper 

 pair of pleurae produced. Lower Silurian, Central Europe, 



Ulcenus Hisingeri, Barr. 



2. P«w6?m6f, Volborth, 1863. Eight body-rings ; short glabella-furrows, and ordi- 

 nary ungrooved pleurae. Lower Silurian, North Europe. 



II. triquetra, Volb. II. Lewisii, Salter. 



3. Bysjjlanus, Burmeister, 1843. Nine rings ; ordinary pleurae. Lower and Middle 

 Silurian. 



//. centrotus, Dalm. II. Bowmanni, Salter. 



4. proper, Dalman, 1826. Ten body-rings ; ordinary pleurae ; eyes subcentral. 

 [The greater number of species belong to this subgenus.] Lower Silurian, universal. 



II. crassicauda, Wahl. II. Murchisoni, Salter. 

 II. Davisii, Salter. 11. Rosenbergii, Eichw. 



5. Eciillanus, Salter, 1866. Ten body-rings; ordinary pleurae; eyes remote and 

 very forward ; glabella-furrows short. Lower Silmian, only British as yet. 



perovalis, Murcb. 



6. Hi/drolcems, Salter, 1866. .Eyes remote, forward; glabella-furrows reaching the 

 front ; head most gibbous. Lower Silurian, Canadian. 



//. conifrons. Billings. 



