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:. The Branchiostegal Rays, 



According to Jordan 1 , the typical number of branchiostegal rays 

 for Amiurus 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 Amiurus catus 

 the usual number was eight. They arise from the posterior (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. The Preoperculum PL II., 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 inter 'operculum. 



6. The Operculum (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 vnteropercuVwm. 



7. The Interoperculum (PI. II., Fig. 1, IOp.) 



Is a short, stout bone, lying between the apex of the operculum 

 and the posterior extremity of the mandible, with which it is united 



i Jo rdan. — Manual of JJ. Amur. Vertebrates, Chicago, 1S76. 



