52 NEW YORK STATE MUSEUM 
observable in many other specimens [as pl. 7, fig. 7]. The oar blade is 
entirely smooth, without scales or hairs. The turning of this oar into a 
vertical position in the backward stroke probably took place mainly between 
the sixth and seventh segments.! 
The material before us is competent to throw some very interesting 
light on the development of this swimming leg. Where typically devel- 
oped, asin Eurypterus, it consists of the large basal segment, which. is 
Figure 11 Ventral surface of male specimen of Callinectes hastatus 
ab, abdomen ;p !—pv, the five pairs of legs (pereiopods); S ITI-VIII, sterna of thorax. (From Brooks) 
followed by two ringlike segments, a longer subtubular segment (the 
fourth), and two shorter segments with triangular section, flat underside, 
sharply keeled anterior edge and expanded distally. These segments 
' Laurie [1893, p. 124] has suggested that ‘it seems more probable, that the foot 
was used for anchoring the animal firmly in the soft mud of the sea bottom, and 
possibly also for shoveling up the sand and mud when the beast wished to conceal 
itself.’’ We shall recur to the question of the use of the last pair of legs in our further 
discussion. 
