DAVENPORT. — EVOLUTION OP PECTEN. 129 



Pecten were more nearly equivalve than their later descendants, but that 

 in very recent times the species has tended to become symmetrical 

 a^ain. Again it is quite possible that the series has a physiological 

 explanation. It would be interesting to know whether those pectens 

 whose lower valve has a breadth under 75 mm. rest on the upper or left 

 valve while the larger ones have the normal position. Whatever the 

 explanation of the series, the main fact stands out clearly that Pecten 

 opercularis is bilaterally unsymmetrical. 



"We have next to consider whether with the loss of symmetry there 

 has also gone a loss of correlation in the variation of the valves. It is 

 perfectly possible, of course, to have perfect correlation in variation of 

 two unequal dimensions. And the two sides of the body in most Bilate- 

 ralia have a very high correlation, as shown below. 



TABLE III. 



Some Coefficients of Correlation of Corresponding Right and Left 

 Organs in Bilaterally Symmetrical Animals. 



R. femur and 1. femur, man 0.9765 ± .0032 Warren, 1898. 



R. tibia and 1. tibia, " 0.9634 -J- .0044 



R. humerus and 1. humerus, " 0.9454 -j- .0091 



R. radius and 1. radius, " 0.9246 -L_ .0151 



R. clavicle and 1. clavicle, " 0.9317 -J- .0115 



Metacarpal 



of thumb on r. and 1. hand, man 0.974 -J- 006 Lewenz& Whitely, 1902. 



of index finger on r. and 1. hand, " 0.990 -£- .002 



of middle finger on r. and 1. hand, " 0.985 -J- .003 



of ring finger on r. and 1. hand, " 0.946 -J- .011 



of little finger on r. and 1. hand, " 0.955 J- .010 



No. r. and 1. pectoral fin-rays, Acerina, 0.700 Duncker, 1897. 



No. r. and 1. pectoral fin-rays, Cottus, 0.700 



No. glands, r. and 1. foreleg, Sus, 0.79 Davenport &Bullard, 1896. 



No. teeth, r. and 1. jaws, Nereis, 0.820 -L_ .008 Hefferan, 1900. 



We have already some data upon the correlation of bilaterally homolo- 

 gous organs occurring in unsymmetrical species. Duncker has made a 

 study of such a case in the flounder, Pleuronectes Jlesics, L. This fish lies 

 upon the bottom on its left side, with both eyes on the right side. In 

 this case the lateral correlation is strikingly small. Thus between the 

 right and left pectoral fin-rays it is 0.590 ± .013 ; between the right and 

 left central fin-rays i< is 0.228 ± .020. Another instance has been afforded 

 by Yerkes (1001) in the case of the fiddler crab, Gelasimus pugilai 

 Latr. These crabs have a symmetrical body except that the first of the 

 walking appendages bears a large terminal claw on the one side and only 

 vol. xxxix. — 9 



