AND THE WEST COASTS OF THE UNITED STATES. 127 



and San Diego forms are more closely similar than those of Tampa and Cold Spring 

 Harbor or even those of Tampa and Beaufort. For the modal number of rays of 

 the Cold Spring Harbor and Beaufort shells is 17, that of Tampa is 20, while that 

 of San Diego is 19. Yet Dall has placed the Cold Spring Harbor and Tampa shells 

 in one species and left the San Diego lot in another. The justification for this is, 

 I presume, the presence of intergrades between the Atlantic Pectens and their 

 absence between those from Tampa and from San Diego. 



VI. METHOD OF MEASURING THE SHELLS. 



To measure rapidly a large number of shells a special apparatus was devised 

 that I have found very convenient. Rising perpendicularly from the periphery of 

 a circular disc of iron ten centimetres thick and planed flat on both sides is a milli- 

 metre scale with a vernier caliper-arm sliding upon it and reaching out over the centre 

 of the plate. On the basal plate close to the inner edge of the vertical scale and per- 

 pendicular to the caliper-arm is drawn the base-line for the horizontal measurements. 

 The plate itself serves as the base for vertical measurements. Upon the basal plate 

 is pasted a piece of millimetre paper ruled in squares, the rulings running respectively 

 parallel and perpendicular to the horizontal base-line. One of the perpendicular 

 rulings of the paper is parallel to the centre line of the caliper-arm and serves as a zero 

 point for measurements of the partial anterio-posterior diameters of the shells, as 

 the horizontal base-line serves as a zero point for the dorso-ventral diameter of the 

 shells. 



The method of using the instrument is this : A single valve is placed on the base 

 with the middle of its beak touching the inner edge of the vertical scale. The hinge 

 is placed parallel to the horizontal base-line. The caliper-arm is lowered until it 

 strikes the shell (at its highest point). The width, anterior and posterior half-length, 

 anterior and posterior half hinge-lengths, and the height of the valve are quickly 

 read and then the shell is removed and the number of internal grooves counted. 

 With a person to record measurements, forty valves may thus be measured and data 

 concerning each recorded in an hour. 



VII. RESULTS. 



The results gained refer to the following characters: 1. Variability in the num- 

 ber of rays; 2. Variability in transverse half-diameter in relation to dorso-ventral 

 diameter; 3. Variability in symmetry of the single valve anterior partial length 



