lvili DESCRIPTIVE REMARKS. 
at an equal distance between the two lobes as in the Herring (Fig. 
15 3). 
A remarkable group, the Cephalaspides includes in Britain alone, 
fifteen species, a number which have been since increased. This group, 
commencing in the Upppermost Silurian with the genus Pleraspis,* 
Plate xxvii, fig. 6, was extensively developed in the Old Red Sand- 
stone. 
With regard to these fossil fish Sir R. Murchison remarks, ‘It 
follows, therefore, that as the grey flag-like strata which pass up into 
reddish beds may either be viewed as the termination of the Silurian 
or the commencement of the Old Red, the genera Cephalaspis and 
Pteraspis are typical both of the uppermost Silurian and the lowest. 
zone of the Old Red or Devonian group.” 
The lowest division of the Old Red Sandstone is characterized in 
Ludlow, &c., and Forfarshire by fish of the genera Onchus, Pteraspis 
(two species), and Cephalaspis (two species), including C. Lyellz, 
Plate xxxiil., fig. 2, a-c. 
In the Middle Division of Herefordshire and part of Shropshire, 
Caithness flags, Orkneys, &c., Scotland, the characteristic fish are 
Coccosteus, including C. decipiens, Pl. xxxiii., fig. 3, a, b. Asterolepis, 
Pterichthys, including P. cornutus, Pl. xxxiii., fig. 1, a, 6. Diplopterus 
Dipterus, Glyptolepis, including G. leptopterus, Pl. xxxiil., fig. 5, a, 6. 
Osteolepis, including O. major, Pl. xxxiii., fig. 4, a, 6; and Holopty- 
chius nobilissimus, Pl. xxxiii., fig., 4, a,b. From strata belonging to 
this division in Britain upwards of seventy species have been already 
obtained. 
The Upper Division has been observed at Dura Den, in Fife, the 
Orkneys, &c., and at Kiltorcan, Co. Kilkenny. The characteristic fish 
are Holoptychius Flemingt, Dendrodus, Pterichthys minor, &e., Onchus, 
and Glytopomus minor ; fifteen species referred to eleven genera have 
been obtained from British strata. 
The majority of these Old Red Sandstone fishes, including the 
figured examples on Pl. xxxiii. of Osteolepis, Glyptolepis, and Holopty- 
chius, with the exception of the anomalous genera Cephalaspis, Pterich- 
thys, and Coccosteus, have been arranged by Professor Huxley into a 
sub-order established by him under the name of Crossopterygidet 
(fringed fin), of which the recent Polypterus of the Nile and other 
African rivers is the living representative. 
The genus Cephalaspis (Buckler-headed), so named from the pecu- 
liar shield covering the head, is represented by a remarkable fish which 
first occurs in the Passage beds from the Silurian rocks into the Old 
Red Sandstone described as C. Murchisoni, from the uppermost bone 
bed near Ludlow. The species we have figured C. Lyelli, Pl. xxxiii., 
fig. 2, is from the Arbroath Paving stone, Glammiss, in Forfar- 
shire. 
* Descriptive remarks, ate p. xlix. + Siluria, fourth edition, p. 260. 
t Men. Geo. Survey, Decade x., p. 24. 
