DEKMAL BOXES OF FISHES. 133 



the liypo-tympanics {ib. 28), which, are attached principally to the 

 pterygoid processes and the back part of the cartilaginous joint 

 of the lower jaw. The mouth of the Sturgeon opens, as in Sharks, 

 upon the under surfjice of the head, and is protruded and retracted 

 chiefly by the movements of the tympanic pedicle, which swings, 

 like a pendulum, from its point of suspension to the post-orbital 

 process. 



The hyoid arch is also small and simple in the Sturgeon. The 

 epi-hyal is short and attached to near the upper end of the hj\}0- 

 tympanic. The cerato-hyal {ib. 4o) of thrice the length, is expanded 

 above, and is attached by ligament extending from that part to near 

 the joint of the lower jaw. The basi-hyal is a short sub -cubical piece : 

 it gives attachment anteriorly to cerato-hyals, and posteriorly to the 

 anterior basi-branchial and hypo-branchial cartilages. 



The three first branchial arches consist of hypo-branchials, pro- 

 gressively decreasing in size, of cerato-branchials, epi-branchials, and 

 pharyngo-branchials : the fourth arch consists of cerato-branchials 

 and epi-branchials : the fifth arch of cerato-branchials only. 



In an old Accipenser Sturio I found the tympanic pedicle in two 

 pieces, and partly ossified. The epi-tympanic was cartilaginous 

 where it articulated with the post-frontal and mastoid, the osseous 

 part commencing at a definite transverse plane. This, as it de- 

 scended, expanded, and reverted to the cartilaginous state, forming a 

 broad triangular flattened plate, which supported the large opercular 

 dermal bone : the hypo-tympanic was a simple strong cylinili-ical car- 

 tilage, giving attachment to the hyoid near its upper end, and to the 

 ligaments suspending the palatine and mandibular arches at its lower 

 end. 



The cartilaginous representation of the par-occipital projects 

 boldly backwards from each angle of the occiput. A triangular 

 supra-scapular cartilage {ib. 5o) has the angles of its base slightly 

 produced, one being articulated to the end of the par-occipital, the 

 other to the ex-occipital region. To the apex is attached the sca- 

 pulo-coracoid arch {ib. 51,52), which is completed below, as in Lepi- 

 dosiren, by ligamentous union, not, as in Sharks, by cartilaginous 

 confluence. The scapulo-coracoid cartilage expands as it descends, 

 sends inwards and forwards a broad wedge-shaped plate, and presents 

 a large perforation at its thick posterior part, answering probably to 

 the perforated ulna of Osseous Fishes, here confluent with the arch. 

 The pectoral fin is articulated to the under part of this perforated 

 projection : the coracoid terminates below by sending inwards and 

 forwards a broad and thin plate beneath the pericardium, which is 

 joined by strong aponeurosis to that of the opposite coracoid. There 



K 3 



