16 MONOGRAPH OF THE FRESH WATER Til. 
preopercular is attached to the medial crest of the epitympanic, whilst the lower 
angle articulates on the body of the articular. 
The preopercular (34), is an exact crescent, on the convexity of which is situated 
the conical spine, directed obliquely upwards. ‘The insertion of the spine is above 
the middle of the crescent. The concavity of the latter is occupied by a thin ex- 
pansion of that bone, making the direct continuation of the tympanic wall. 
The opercular (35), is articulated to the posterior articulating process of the 
epitympanic. It is irregularly triangular like the hypotympanie and formed like- 
wise of an articulating head at its summit, and of two branches, between which is 
a thinner expansion of that bone. 
The subopercular (36), is provided at its lower edge with a spine whose point is 
directed downwards and forwards, a little more conspicuous than in the figure. 
That bone sends off two branches, a spiny one directed upwards which unites 
with the inner edge of the opercular, and another thin and membranous, running 
all along the posterior and free edge of the same bone, and in fact, constituting the 
actual edge of the opercular. 
The interopercular (37), is an elongated and narrow piece, very uniform in shape, 
pointed forwards, truncated behind, extending from the subopercular behind, to the 
hypotympanic and articular, in front. In a fleshy specimen it is not apparent, 
being covered by the lower branch of the preopercular. When the skin is removed 
its edge may then be seen. 
Amongst the bones of the head and face there still remain to be mentioned 
those which have been classified under the special denomination of dermo-skeleton. 
To the dermo-skeleton belongs the lateral line which is described below in § 9, 
and figured on Plate II, Fig. 11. When the lateral line enters the head, it passes 
under the suprascapular into the mastoid groove, the posterior portion of which is 
covered by two thin and small bones, our swpramastoids (8' 8'). From the mastoid 
groove a cartilaginous or bony tube composed of two pieces, rans downwards into 
the preopercular. These two tubular pieces we call the swpratympanics (72, 72’), 
being the supra temporals of Prof. Owen. 
In advance of the mastoid groove, and above the epitympanic, there are two 
other small tubular and horizontal pieces, our supralachrymals (73% 73°), of which 
the lachrymal (73), is the direct continuation downwards. The latter forms the 
posterior edge of the orbit. 
The suborbitals (73'), are two in number; the posterior one is placed obliquely 
across the cheek, from the preopercular spine to the orbit, where it meets the lach- 
rymal tube, which passes directly into the anterior suborbital, forming the inferior 
edge of the orbit. In advance of the latter, and near the centre of the snout, the 
turbinal (19) may be seen, small, irregular, with acute angles. Between the latter 
and the prefrontal there exists a very small, insignificant swpraorbital (71). 
The hyoidean arch is represented in Fig. 5°. The stylohyal (38) is only half 
ossified ; it is slender and elongated, resting upon the epihyal. The epihyal (39), 
is broad, short, and subtriangular, giving points of attachment to two of the branchi- 
ostegals. The ceratohyal (40), is the largest bone of the hyoidean arch, elongated, 
