NATURAL HISTORY OF AMERICAN LOBSTER. 



261 



be: 1+1+2+4=8, or, designating each spine by its serial number as in table 6, 

 I :4 :3 :4 :2 :4 :3 :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 t he 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. i, 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° 

 with the lock spine, which is nearly 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 aUgnment 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. — Bight great cheliped of fourth stage lobster, from 

 above, showing upper hinge process (« k p) of carpus, and disar- 

 ticulated ischium with interlocking process (j-'), and future inter- 

 locking spur (^), which is rudimentary. Compare text figure 8, 

 and plate xxxiv, figures x, 3, and 4. 



