THE ANATOMY OF THE THYROID GLAND 157 



gated form of the outward mass is flattened, transversely elon- 

 gated, and of the same peculiar triangular or shield-like shape 

 which is characteristic of the organ in the dogfish and closely 

 simulated by that of Raia. It seems possible that the elongated 

 gland observed by De Meuron in Scyllium might be susceptible 

 to a similar analysis. 



Guiard ('96) studied six species of the Selachii and five of the 

 Batiodei. In Scyllium he found the thyroid gland of pyriform 

 shape, the anterior extremity being prolonged forward as far as the 

 anterior margin of the lingual cartilage ("copule"), where it passes 

 between the two lateral halves of the coraco-hyoid muscle. This 

 description, as given by Guiard, corresponds with the position and 

 form of the gland which I find in Carcharias and which, as regards 

 the anterior prolongation, appears to be analogous to the pyrami- 

 dal process of mammals. But Guiard's fig. 1, in the absence of 

 specific contradiction in his text, might be taken to indicate that 

 the thyroid gland had been found beneath the coraco-hyoid 

 muscle; this is not the case in any of the species which I have 

 examined and I presume it is not the case in Scyllium catulus, from 

 which species his figure was drawn. In each species I have found 

 the gland lying, without exception on the ventral surface of the 

 coraco-hyoideus, between it and the coraco-mandibularis, except 

 that at the anterior portion the gland lies between the coraco- 

 hyoid muscles of the two sides, the divergence of the two muscles 

 exposing the ventral surface of the cartilage at this point. In the 

 Batoideithe coraco-hyoidei are so widely separated that the whole 

 thyroid gland may come to lie directly upon the basi-hyal cartilage, 

 the aortic bifurcation and the coraco-branchialis muscles, 

 which are successively exposed from before backwards by the 

 separation of the coraco-hyoids, but in this case the fascia which 

 covers the ventral surface of the coraco-hyoids dips beneath the 

 dorsal surface of the thyroid gland. 



In Acanthias vulgaris and Mustelus loevis Guiard as did De 

 Meuron, notes the tendency of the thyroid to present detached 

 vesicles, its contour being very irregular. In Galeus canis the 

 thyroid gland lies rather farther forward and is partially covered 

 by a fold of the buccal mucosa. In Carcharias glaucus the 



