16 MONOGRAPH OF THE FRESH WATER III. 



preopercular is attached to the medial crest of the epitympanic, whilst the lower 

 angle articulates on the body of the articular. 



The jyreopercidar (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 hypotympanic 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 suhopercular (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 suhopercular 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 hane^, owx supraniastoids (8' 8'). From the mastoid 

 groove a cartilaginous or bony tube composed of two pieces, runs downwards into 

 the preopercular. These two tubular pieces we call the supraiympanics (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 suprcdachrymals (73" 73''), of which 

 the lachrymal (73), is the direct continuation downwards. The latter forms the 

 posterior edge of the orbit. 



The suhorhitals (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 

 turhinal (19) may be seen, small, irregular, with acute angles. Between the latter 

 and the prefrontal there exists a very small, insignificant supraorhital (71). 



The hyoidean arch is represented in Fig. S*". 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, 



