820 REPOET OF COMMISSIONEE OF FISH AND FISHERIES. [74] 



curved brancliiostegal rays, wliich in life are held in this position by lig- 

 ament. The hinder and larger one is possessed of quite a blade-like 

 extension, and the bone, like the rest of the series, is gently curved up- 

 wards. Two more branchiostegal rays are attached in a similar manner 

 to the ceratohyal, the four bones being placed at about equal distances 

 apart. The series of branchiostegal rays progressively increase in size 

 from before backwards, the anterior ones being the most abruptly 

 curved. The next two rays in order are attached to the lower margin 

 of the ceratohyal, and I am under the impression that I have dissected 

 specimens where a seventh ray has existed that was attached in order, 

 beyond these, just within this border. As we know, the branchiostegal 

 rays support a membrane of the same name, which forms sort of an 

 auxiliary gill-flap. 



Lying in the median plane, posterior to, but attached by ligament to 

 the symphysis of the cerato-hyals, we find a plate of bone, that in the 

 living fish separates the sternohyoid muscles. This bone is of a tri- 

 angular outline, with its apex forward, a part of which bears a dilata- 

 tion and superior osseous loop for a greater ligamentous attachment. Its 

 lower margin is transversely expanded, and the plate is further strength- 

 ened by the development of an osseous rod that runs longitudinally 

 through its center. This azygos plate is the urohyal, and is peculiar to 

 fishes. In life it lies between the sternohyoid muscles, and is not al- 

 ways present where a glossohyal exists. 



Aside from this urohyal and the branchiostegal rays, the bones we 

 have been thus far examining constitute the hyoid arch, and this Bass 

 presents it in what may be said a typical form for fishes, if anything can 

 be adopted, as a standard in form in a class where all the structures 

 vary so in shape. 



The relation of the various bones of the piscine skull and their func- 

 tions, when we come to compare them with the homologous elements in 

 the higher animals — man, for instance — has always presented to my mind 

 one of the most interesting subjects in anatomy. Here in our specimen 

 we have the hyoid arch, supporting, on either side, a series of branchi- 

 ostegal rays. These rays constitute the siceleton of an organ of defense to 

 the respiratory apparatus. It is believed by some that the opercular 

 bones are modified branchiostegal rays, and these in their turn form 

 the lateral osseous wall of defense to the gill chamber, also the respiratory 

 apparatus. The operculum articulates with the hyomandibular of the 

 suspensorium, which bone is said to be the representative of the incus 

 of the human ear, while the lower bone, the quadrate, of the suspenso- 

 rium, is a segmented portion of the malleus, another of the auditory os- 

 sicles in man. IN"ow, in its turn the quadrate articulates with the man- 

 dihle or lower jaw, a bone in one way subservient to the digestive appa- 

 ratus. 



Lying in the angle formed by the limbs of the hyoid, we find the 

 branchial arches. The arrangement of these in Micropterus is so like it 



