42 



its inner side is excavated for the insertion of the pointed end of the mesotympanic, and the 

 anterior angle is wedged between the pretympanic and the pterygoid (24), and is firmly united 

 to the latter : the trochlea is slightly concave transversely, convex in a greater degree from 

 before backwards. 



The mandible or lower jaw (29, 32) is the lower portion of the tympano-mandibular arch, 

 being articulated to the hypotympanics above, and closed by a ligamentous union or bony 

 symphysis with its fellow at its lower end. The term ' minus ' is applied in Anthropotomy 

 to each half of the mandible, and each ramus consists of two, three, or more pieces in differ- 

 ent fishes. In most fishes it consists of two pieces, one (20) articulated to the suspensory 

 pedicle, and edentulous, and the other (32) completing the arch, and commonly supporting 

 teeth. In the Cod and some other fishes a third small piece (30) is superadded, at the angle 

 of the posterior piece. 



The diverging or radiated appendage of the tympano-mandibular arch consists of the bones 

 which support the gill-cover, a kind of short and broad fin, the movements of which regulate 

 the passage of the currents through the branchial cavity, by opening and closing the branchial 

 aperture on each side of the head. The first of these opercular bones, which forms the chief 

 medium of the attachment of the appendage to the supporting arch, is the preopercular (34), 

 which is usually the longest in the vertical direction, if not the largest of the bones : it is 

 here, in the Cod, bifurcate above, and the lower slender angle is continued downwards and 

 forwards to beneath the hypotympanic. Three bones usually constitute the second series of 

 this appendage : the upper one is commonly the largest and of a triangular form, thin, and 

 with radiated lines like a scale; it is the 'opercular bone' (35). Below this is the suloper- 

 cular (39). The lowermost bone, called the interopercular (37), is articulated to the preoper- 

 cular above, to the subopercular behind, and usually to the back part of the mandible ; it is 

 attached also, in the recent head, by ligament to the ceratohyoid in front. 



The third inverted arch of the skull is the ' hyoidean,' and is suspended, in Osseous 

 Fishes, through the medium of the epitympanic bone, to the mastoid. The first portion of 

 the arch, called stylohyal (M), is a slender styliform bone, which is attached at the upper end 

 by ligament to the inner side of the epitympanic, close to its junction with the mesotympanic, 

 and at the lower end to the apex of a triangular plate of bone, which forms the upper portion 

 of the great cornu. To this portion is assigned the name of epihyal (39). The third longer 

 and stronger piece is the ceratohyal (40). The key of the inverted arch or body of the hyoid 

 is here formed by four small subcubical bones on each side, which bear the name of basi- 

 hyals (41). The triangular flattened bone, which expands as it extends backwards, in the 

 middle line, from the basihyals, is the 'urohyal' (43). 



The diverging appendages of the hyoidean arch have the form of simple, elongated, slen- 

 der, slightly curved rays, articulated to depressions in the outer and posterior margins of the 

 epi- and cerato-hyals : they are called ' branchiostegals ' or gill-cover rays (44), because they 

 support the membrane which closes externally the branchial chamber. 



The fourth cranial inverted arch is that which is attached to the paroccipital and mastoid 

 in the naturally articulated skull of the Cod. The superior piece of the arch, svprascapula (50), 

 is bifurcate ; one prong was attached anteriorly to the paroccipital, the other and shorter 



