54 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
Visceral Skeleton. 
The visceral skeleton consists morphologically of the suspensorium, 
the mandible, and the hyo-branchial apparatus. The first of these 
parts has been already considered with the skull. 
Mandible. 
The mandible consists of two curved cartilaginous rods (Meckel’s 
cartilages), joined anteriorly and diverging posteriorly, giving it an 
arched shape, and bony plates of dermal origin, three (the dentale, 
the angulare, and the spleniale) encasing each cartilage. 
Meckel’s cartilage is anteriorly a cylindrical rod closely joined to 
the cartilage of the other half of the mandible. Posteriorly it is 
flattened laterally and widened, its dorso-ventral axis becoming three 
or four times its original length. The postero-dorsal angle projects 
slightly and is rounded, articulating with a depression on the distal 
end of the quadrate. 
Of the bony plates, the dentale is the largest, covering the outer 
surface of the anterior two thirds of the cartilage. Its outer surface 
is curved inward dorsally and ventrally, so that the inner surface 
has a marked groove in which the cartilage lies. On the inner sur¬ 
face of the anterior half there is a single row of from twenty to thirty 
small pointed teeth which project about one half their length beyond 
its dorsal border. Posterior to this, the dorsal border slopes in a 
ventral direction forming a very acute angle with the ventral border. 
Fitting close to the cartilage, on the posterior half of. its inner 
surface, is the angulare. This bone is a flat splinter, the posterior 
half being about half the depth of the cartilage, while the anterior 
portion tapers to a point which reaches nearly to the posterior tooth 
of the dentale. It bears no teeth. 
The spleniale, the third bony plate, is an extremely minute scale 
bearing dorsally six or eight teeth which are continuous with the 
row of teeth on the dentale. It lies against the inner dorsal sur- 
face of the cartilage, its whole ventral border contiguous with the 
dorsal border of the angulare at the posterior part of its anterior 
third. The bony case of the cartilage is therefore not complete, for 
the posterior third of the cartilage is not covered on its outer sur¬ 
face nor on its inner surface dorsally. The cartilage in this region 
seems especially thick and tough. 
