256 
BULLETIN OF THE BUREAU OF FISHERIES. 
the lower hinge process of the carpus. It follows from these relations that the articular 
surfaces of the carpus face, while those of the claw look in opposite directions. 
This remarkable joint suggests the hinge of an ordinary folding pocket rule, but with 
a different locking device. It is neither a true pivot, tenon-and-groove, or ball-and- 
socket joint, and so far as I am aware its principle is not found embodied in any of 
the common mechanical devices. We find it well developed at the fourth stage, with 
little later change except in the further overgrowth of the hinge processes. (Fig. 9.) 
Such a joint works with great precision in its prescribed plane, with little or no appreci- 
able lost motion, and would seem to be an adjustment by means of which the big claw 
is firmly secured to the supporting carpus, and the voluminous flexors of this segment 
can react upon the great weight of the claw to the best advantage. 
In the crayfish ( Cambarus ) the big claw is not locked to the carpus, but moves loosely 
on double hinges of the typical ball-and-socket order, each hinge consisting of carpal 
ball, and propodal socket mounted 
on a round tubercle. In Callinectes 
and certain other Brachyura exam- 
ined (text fig. 5) the great cheliped 
has suffered little or no torsion, 
and the dactyls open upward as 
in the larval lobster. The claws 
move on modified ball-and-socket 
hinges, which are firmly locked to 
the claw but in quite a different 
manner from that of the lobster. 
The propodus in this case bears 
cups [l h (socket) fig. 5) on both 
upper and lower sides, which are 
locked over the balls by processes 
(u and l h p) growing out from this 
segment and not from the carpus. 
The crab’s claw thus swings vertically in and out through an angle of upwards of 90°. 
While the locked, sliding joint of the lobster, particularly in the reversal of its 
hinges, suggests the ordinary ball-and-socket device of the other limb segments, and 
even more that of the crab’s chela, it would be difficult to decide whether one was 
better from a mechanical standpoint than the other, or to imagine how either could 
have arisen from the simpler type upon any principle of selection. 
ASYMMETRY IN THE BIG CLAWS OF THE LOBSTER. 
The marked dissimilarity of the big claws (pi. xxxvii) in regard to both their 
structure and chief functions in all lobsters above an inch or an inch and one-half long, 
has led to various distinctive names on both sides of the Atlantic. Fishermen often 
speak of the “knobbed” and “quick” claws. The larger is adapted for crushing the 
Fig. 5. — Locked sliding joint of big claw of crab ( Callinectes hastatus ); 
in same plane as represented in figure 3, showing modified balls and 
sockets, but with no reversal on upper and lower sides; hinge processes 
(w and l k p) here arise from the propodus of claw. 
