AND THE WEST COASTS OF THE UNITED STATES.' 125 



of the cases is there one more rib behind it than in front; in 6% there is the same 

 number. The anterior exceeds the posterior number of ribs 



by one in 36% of all cases; 

 by two in 39%; 

 by three in 16% ; 

 by four in 1%. 



Thus, typically, the shifting of the white rib forward by one would place it in the 

 middle of the series. In addition to the middle white rib nearly all of the upper 

 valves show a trace of a white rib lying to one side of the median or of two ribs lying 

 one on either side. Of these the anterior rib is the more constant. In the majority 

 of cases, 72%, this rib is separated from the middle white rib by four ribs; in 22% 

 by three ribs; in 6% by five ribs. When the posterior white rib is present it is usually 

 separated by three ribs from the median white rib (84% of all cases); more rarely 

 by four ribs (16% of cases). The white ribs, either median, anterior, or posterior, 

 may be uniformly light, but very frequently there are only patches of white which 

 give the whole shell a mottled appearance. These light-colored ribs, most frequently 

 the middle one, are found in the Pectens irradians from Cold Spring Harbor in corre- 

 sponding positions. So they are characteristic of the East Coast species. Espe- 

 cially important for our purpose is the fact that, while these white rays are not a 

 common phenomenon in the genus Pecten, they occur also in the lot from San Diego. 

 Here the number of ribs anterior to the median white rib exceeds the number pos- 

 terior to it in every case; the excess is one rib in 47% of the cases; two ribs in 47%; 

 and three ribs, for instance, 11:8, in 6%. The lateral ribs are much less constant in 

 occurrence than the middle one, being distinguishable in only about 10% of all left 

 shells, and then almost exclusively anterior to the middle rib. The number of ribs 

 between the anterior and the median light-colored rays is modally four. Here, again, 

 the white rib is rarely completely colorless, but there are frequently dark bars across 

 it giving, again, a mottled appearance to the shell. 



Now, the foregoing resemblances in the details of color markings cannot be acci- 

 dental; they point to a blood-relationship of the Gulf and San Diego forms. 



3. General Proportions — In both lots of shells, as in general in Pecten, the 

 posterior partial length exceeds the anterior.* The excess is only about 6% in the 

 Tampa form; it is 15% in the San Diego form. But the skewness of even the San 

 Diego shell is slight as compared with that of some other Pacific Coast species, for 



* That part of the anterioposterior diameter of the valve that lies in front of the plane passed perpendicular 

 to the hinge at the beak may be called the anterior partial length; the remainder is the posterior partial length. 



