766 J. F. GUDERNATSCH 



the organ. Yet on the other hand, there can hardly be found a 

 group of foUicles detached from the main thyreoid body. 



The isthmus is long, as in Brevoortia, and hence the thyreoid 

 region is much elongated (pi. I, fig. 10), measuring 4 cm. in length. 

 It is not, however, as narrow as in the menhaden, having a wide 

 lateral extension. The anterior end is pushed far forward, 2.5 cm. 

 in front of the aortic bifurcation, so that it also comes to lie in front 

 of the hyoid arch. The entire development of the organ takes 

 place more cephalad than usual and the main mass lies in front of 

 the aortic bifurcation (sharks!), deeply buried in the body of the 

 tongue, as a consequence of the ventral extension of the copulo- 

 hyoid (fig. 12, A). It occupies a more ventral position than any 

 other fish thyreoid. The follicles are located around a large vein 

 and are rather closely arranged. As the basi- and hypohyalia 

 retract the follicles creep into the clefts between them and thus 

 the thyreoid mass assumes the shape of a horse-shoe, the two 

 arms of which point dorsally (fig. 12, B, C). The smooth muscle 

 fibres of this region are completely invaded by follicles (pi. IV, 

 fig. 21), as are also the bones of the gill arch, especially the copula, 

 in regions where they lose their compactness and break up into 

 lamellae. The thyreoid takes the form of three masses converging 

 ventrally, and as we pass back it expands more and more on the 

 sides, 6 to 7 mm., while the median branch becomes smaller. 

 About one cm. in front of the aortic bifurcation the most extensive 

 region of the gland is reached. In cross section the mass is rhom- 

 boidal, the diagonals being about 7 and 4 mm. The lateral exten- 

 sion decreases while the ventro-median mass increases, from which 

 two branches tend dorsally along the edge of the copula. Thus 

 again the sections show a horse-shoe shape, with a broad middle 

 piece and narrow dorsally converging arms, in which the follicles 

 are oval with their longer axis parallel to the line of extension. On 

 reaching the first branchial arteries, which run in this species 

 towards a ventro-lateral zone and do not come into contact with 

 the follicles (fig. 12, C) we pass to their union where a few follicles 

 surround them (fig. 12, D). The central portion of the gland 

 becomes smaller and lies separate in the aortic bifurcation while 



