208 DR OTTO THILO. ^ 



they were formed of metal, whereas for copies in wood, a 

 circular profile is sufficient; its middle point lies excentric, 

 fig. 13, M. 



In the frictional stops which are in use in mechanics the 

 pawl shows a partly involute, partly circular, profile {vide 

 Eeuleaux, Kronstructcur, p. 639). 



In the different kinds of Monacanthus the length of the click 

 bone is different. In some the upper end of the click bone 

 is very long (fig. 11), whereas in others it is totally absent, there 

 being only a small round bone : that shows that the click bone 

 formerly had the form of an ordinary fin ray, and by degrees 

 degenerated so much that only the end of the joint remained. 

 In some kinds of fish we find a collection of fin rays all helping 

 to hold up the front fin spine, as in the herring king (Zeus). 

 The two front spines of this fish are, as in fig. 2, displayed 

 shortened. One sees that the front spine leans against the 

 second by means of a spur-like elongation when it is quite 

 raised. The click or stop can only be brought into play by 

 raising the fin, whereas in the clicks of Monacanthus the fin 

 can be fixed under any angle between and 90. 



In addition to the triple-pronged mechanism there can be 

 added those similar to the serpent's fangs (figs. 3-5). 



At first sight, it would almost appear as if a greater number 

 of members were necessary to apply the stop mechanism here, 

 but a closer examination shows that only one part of all those 

 members comes into play. According to diagrams 4 and 5, 

 there are shown four bones which are jointed and connected 

 with the skull. 



1. The skull is considered stationary. 2. The upper jawbone 

 with fang. 3. Pterygoid. 4. Quadrate. 5. Temporal bone. 

 These five bones form a five-jointed endless chain. 



Five members are absolutely necessary, so that when raising 

 the fang the pterygoid is not separated from the skull, but 

 rather lies as close as possible against it. If there were only 

 four members, 3 would stand out considerably from 1 when 

 the fang is raised in biting — {a) through the pressure to be 

 overcome by the bite, (6) through the pressure against the fang. 

 This danger of breaking is removed by the introduction of 

 member 5. Besides, the strength of resistance of 3 is annulled 



