S22 REPOET OF COMMISSIONER OF FISH AND FISHERIES. [76 | 



zontal portion, of course, looks almost directly upwards. The upper 

 aspect of this plate is marked by wavy lines, five or six in depth, that 

 run round the bone parallel to its outer margin. Passing obliciuely 

 through the center of the bone is a mucus canal, the posterior opening 

 of which is a circular foramen placed at the back of the articular pro- 

 cess. The anterior opening is flattened and is opposite a similar canal 

 that passes through the body of the dentary. The vertical portion of 

 the articular is of a triangular form, and contains, in a canal in its sub- 

 stance, open on the inner aspect, running longitudinally at its base, the 

 Mecklian cartilage (Fig. 15 M.c). This cartilage passes into the dentary 

 which ensheaths it nearly to the symphysis. The posterior border of 

 the vertical plate of the articular is re-enforced by a thickened and 

 raised rim, the laminated portion being beautifully marked by white 

 lines running parallel to its superior margin. Eadiating lines are also 

 carried out to this border from the angle formed by its thickened pos- 

 terior border and its line of union with the horizontal portion. 



The inner posterior angle of the horizontal portion of the articular is 

 completed by a separate piece of bone. This is the angular. It is tri- 

 angular in form and unites with the articular in a roughened suture. 

 This union is not so firm but that the piece comes away during ordi- 

 nary maceration. 



The two dentary pieces join each other anteriorly in the median plane 

 in quite a firm symphysis. Thus formed, the entire bone constitutes 

 the major part of the mandible, its superior border being thickly stud- 

 ded with rows of teeth. These rows become fewer in number, and 

 the teeth progressively smaller as we proceed backwards, and they 

 cease to appear within short distance of the posterior projections be- 

 hind. Each dentary element, posteriorly in the vertical plane, is deeply 

 notched by a triangular indentation (Fig. 15). At the anterior apex 

 of this triangle enters the Mecklian cartilage. The limb below, of this 

 fork, lies in the horizontal plane, constituting the hinder half of an ellip- 

 tical plate of the dentary, similarly situated. It is through this part 

 that the mucus canal is ensheathed, and into it on the inferior surface 

 open three foramina placed a short distance apart. Other foramina 

 pierce each dentary element on the outer aspect, half way between the 

 symphysis and the apex of the postero- superior process. They are for 

 the passage of vessels and nerves. 



Huxley, in speaking of the mode of development that takes place in 

 this region, tells us that " two ossifications commonly appear near the 

 proximal end of Meckel's cartilage, and become bones movably articu- 

 lated together. The proximal of these is the quadrate bone found in 

 most vertebrates, the malleus of mammals ; the distal is the os articulare 

 of the lowe^ jaw in most vertebrates, but does not seem to be repre- 

 sented in mammals. The remainder of Meckel's cartilage usually per- 

 sists for a longer or shorter time, but does not ossify. It becomes sur- 

 rounded by bone, arising from one or severaL centers, in the adjacent 



