107 



coraco-mandibularis muscle and covered on its ventral side by a me- 

 dian portion of the same muscle. The gland varies somewhat in shape 

 and may extend as far forward as the foramen. 



In a few cases the gland itself appears to be inserted 

 into the foramen. In most cases, however, the connective tissue in- 

 vestment can be traced forward as a strand of tissue distinct from the 

 fasciae of the surrounding muscles, becoming inserted into the foramen 

 as a sort of plug. In no case has there been found any connection 

 between this connective tissue and the mucous membrane covering the 

 dorsal surface of the basihyal. 



Sections have been made of portions of the basihyal cartilages 

 containing the foramina in question. In the case of two examples of 

 S. canicula out of nine examined, and one of S. catulus out of three 

 examined, a very interesting condition has been found. Follicles, 

 identical in appearance and structure with those of the thyroid gland, 

 are present in the connective tissue occupying the foramen (Fig. 4). 



c. 



ty. 



Hi.fc 



Fig. 4. Transverse section of basihyal, passing through foramen, of Scyllium 

 canicula X 27. b.v. blood vessels, c. cartilage of basihyal. c.t.p. connective tissue 

 plug in foramen, th. fo. thyroid follicles. 



The occurrence of these follicles in this particular region, and 

 the fact that the connective tissue investment of the thyroid gland 

 only extends as far forward as the foramen and becomes inserted into 

 it, lead me to the conclusion that the latter marks the situation of 

 the anterior end of the original evagination from the ventral wall of 

 the oral cavity; the cartilage of the basihyal having been laid down 

 around it. 



The foramen corresponds in position with that which I have de- 

 scribed as occurring in the basihyal of Chlamydoselachus which is oc- 

 cupied by the vestige of the thyroid duct. Moreover the thyroid fol- 

 licles which occur in the foramina of these two species of Scyllium 

 can only have been formed from endodermic tissue, as is the case in 



