BY R. ETHERIDGE, JUNR., AND JOHN MITCHELL. 507 
lenses in a vertical row appears to be normal for the examples 
from the Middle Trilobite Bed; but a specimen from the Upper 
Trilobite Bed has thirteen to fourteen lenses in the central rows. 
The tail spine is anchylosed to the border, and when the 
border and spine are removed a short dagger-like extension of 
the axis is exposed, such as is shown in most of the figures of the 
European H. caudatus. It is in this condition that the tail of 
our species bears a strong resemblance to H. caudatus; but what- 
ever may be the case in the latter, it is, judging from the evidence- 
furnished by a large number of specimens, almost certain that, in 
every instance where the tail of our species exhibits the short 
deltoid form of spine, the true spine has suffered removal. 
We believe the forms figured by McCoy from the Victorian 
Upper Silurian as Phacops ( Odontochile ) caudatus to be the same 
as our H. meridianus. His figures show the much longer eye 
and multisegmented pygidial axis; but McCoy's glabellae are 
granulate. H. meridianus, both as regards the N.S. Wales and 
Victorian specimens is so finely granulate throughout as to be 
practically smooth without a lens. 
Touching the relation of our species to the typical European 
H. caudatus, Brim., the eyes are proportionately further forward 
in H. meridianus, the palpebral lobes and genal lobes wider trans- 
versely, and there is no neck tubercle. The eyes are less lunate, 
or arched in contour, and consequently longer fore and aft, and 
the surface of the glabella non-tuberculate. The pygidia differ in 
the excess of segments over those of H. caudatus, possessing 
seldom less than sixteen in the axis of the smaller pygidia, and 
usually eighteen or nineteen exclusive of the terminal appendage. 
Victorian and N.S. Wales specimens agree in this. Our form is 
also long tail-spined when perfect, thus resembling //. longicau- 
datus, but unlike the latter we have never seen an individual 
bearing a frontal spine. As regards the form of the glabella, H. 
meridianus seems to come nearer to H. longicaudatus. The genal 
spines are the same length in both the European and Australian 
forms. //. caudatus occasionally has a granulated pygidium axis, 
