170 JEREMIAH S. FERGUSON 



between this point and the inferior mandible. The cartilages 

 of the hyoid arch except only the dorsal surface of the basi-hyal, 

 are loosely adherent to this membranous floor •so that when 

 the mouth has been closed and the pharynx contracted the 

 tongue-like basi-hyal cartilage is pushed forwaid, producing a 

 deep fold in the oral mucosa. A needle thrust through this fold 

 in the median line, passing dorsal to the cartilage, penetrates 

 directly into the thyroid gland. 



Not all of the ventral surface of the basi-hyal cartilage is 

 covered by the insertion of the coraco-hyoideus muscle ; the por- 

 tion of the cartilage thus exposed varies in different species and 

 to some extent in individuals. In Carcharias and Raia the muscle 

 is inserted into only a small portion of the cartilaginous surface, 

 while in Mustelus all but a narrow anterior margin is covered by 

 the muscle fibers. The thyroid gland typically lies upon this ex- 

 posed cartilaginous surface, extending backward for a greater or 

 less distance upon the ventral surface of the coraco-hyoideus 

 (fig. 1, C). In Mustelus and Squalus the ventral surface of the 

 basi-hyal cartilage has a raised arciform anterior margin with 

 a very slight medial depression, in Raia it is nearly flat, and in 

 all these species the thyroid gland overspreads the cartilage like 

 a thin membrane whose convex anterior border nearly corres- 

 ponds with the outhne of the cartilage; in Carcharias the cartilage 

 presents a deep median groove or furrow into which the thyroid 

 gland sinks, lying there in a gelatinous mass of connective tissue 

 so voluminous that the gland is partially, sometimes wholly, 

 obscured. 



In the dogfish and shark the thyroid gland is rarely pushed for- 

 ward beyond the margin of the basi-hyal cartilage; in Raia the 

 organ may extend farther forward so that it rests in part upon 

 the membranous floor of the oro-pharynx. In one of the skates I 

 found the gland carried so far forward that it lay wholly in front 

 of the cartilage. In Mustelus and Carcharias individual variations 

 are much less frequent than in Raia, but in Raia in the majority 

 of individuals the gland lies directly upon the bifurcation of the 

 ventral aorta (fig. 7). 



