eT —— = = a 
THE OSTEOLOGY OF AMIURUS CATUS. 289 
plate, bearing on its upper surface a high longitudinal keel which 
bifurcates anteriorly, each division continuing its way upon the 
anterior round portion, diminishing as it passes forward. Upon the 
upper surface of the flattened portion, and separated from each other 
by the median keel, are the two hyo-clavicular muscles. 
4. Tue BrRaANCHIOSTEGAL Rays, 
According to Jordan’, the typical number of branchiostegal rays 
for Amiwrus is nine, varying, however, from eight to eleven. The 
variation seems to occur even individuals, there being, for instance, 
sometimes nine on one side and eight on the other. In Amiwrus catus 
the usual number was eight. They arise from the postericr (inferior) 
borders of the epihyals and ceratohyals, which possess notches for 
their articulation. The inner ones are short and rounded, but the 
outer (superior) ones are more or less flattened, the last two being 
quite flat and applied to the under surface of the operculum. In 
fact I would prefer to state the number of the rays at seven, consider- 
ing the upper one as the suboperculum. 
5. Toe Preopercutum ,Pl. I1., Fig. 1, PrOp.) 
Is more or less firmly united with the hyomandibular and quad- 
rate. It is broader at the lower part than above, and is grooved on 
its anterior border for the reception of the lower part of the 
hyomandibular, the symplectic, and the quadrate. It is a continua- 
tion of the longitudinal flattened ridge of the hyomandibular and 
contains a mucous-canal, foramina upon its surface being for the 
exit of branches to the pores. Behind and below it rests upon the 
operculum and imteroperculune. 
6. THe Oprrcutum (PI. II., Fig. 1, Op.) 
Is a triangular scale-like bone, articulating with the knob on the 
hyomandibular. Its apex is in relation to the interoperculum. 
7. Tue Inreropercutum (PI. IL, Fig. 1, /Op.) 
Ts a short, stout bone, lying between the apex of the operculum 
and the posterior extremity of the mandible, with which it is united 
1 Jorden.—Manual of N. Amer. Vertebrates, Chicago, 1876. 
