NATURAL HISTORY OF AMERICAN LOBSTER. 
26l 
be: 1 +1 +2+4=8, or, designating each spine by its serial number as in table 6, 
1 :4 :3 14 :2 14 13 :4=8. 
About midway on the dentate margin of the “hand” (fig. 12 and 13) or propodus 
one finds a stout spur which I shall call the “lock spine” (L in all the figures). As 
we shall see, it is really a displaced spine of the first order. It fits into a shallow groove 
of the dactyl, which is often slight or 
wanting, and forms the lock of the claw. 
Upon closing, the dactyl falls on this 
spur, and, its teeth sliding under those 
of the opposed jaw, it is firmly locked 
in this position, so that no lateral mo- 
tion is possible. (Fig. 1, pi. xxxvi.) 
To complete this adjustment, the tips 
of the forceps are bent like the man- 
dibles of a crossbill, the dactyl under- 
lapping. The spines of the propodus 
are bent upward, those of the dactyl 
downward so that in the claws of some 
individuals they make an angle of 45 0 
withthe lock spine, which isnearly ver- 
tical. Moreover, the spines are aligned 
very accurately, and in a peculiar man- 
ner. The spines of the “upper jaw” 
or propodus are all tangent to a line 
traversing its lower border, while those 
of the dactyl or underlapping jaw meet 
a line drawn along its upper margin. 
This reversal of the alignment it will 
be observed makes it possible com- 
pletely to close and at the same time to 
lock fast the jaws of an instrument 
having this structure. It follows that 
the teeth do not interlock but overlap 
(fig. 12 and 29). 
The tendency of the spines to in- 
crease in geometrical ratio is often 
present and if effective would in the 
next progression give a period of 16 
spines. Under these conditions the periods are generally incomplete, seldom yielding 
over 13 spines. 
The formula given above seldom holds good for more than two or three periods, 
and in many claws no period is quite perfect. At both proximal and distal ends of the 
series the periods become irregular and the identity of the spines is lost. Some means 
Figs. 9 and 10. — Right great cheliped of fourth stage lobster, from 
above, showing upper hinge process (u h p) of carpus, and disar- 
ticulated ischium with interlocking process (s' 1 ), and future inter- 
locking spur (s 3 ), which is rudimentary. Compare text figure 8, 
and plate xxxiv, figures 1,3, and 4. 
