126 PROCEEDINGS OF THE AMERICAN ACADEMY. 



niag ") off the Hebrides and Faroe Islands at a depth of 670 metres. 

 In the Mediterranean it occurs from Gibraltar to Asia Minor on the 

 coasts of Europe, Africa, and Asia, and most of the isles, also in 

 the Adriatic and iEgean seas. The species is found fossil from the 

 Miocene on. 



III. Methods. 



The two valves of Pecten are respectively right and left ; they are 

 also upper and lower, for the mollusc, during early life at least, lies 

 upon its right side. The right valve is notched near the hinge to 

 permit the passage of the byssus, the tough, threadlike secretion by 

 which the shell is attached to the substratum. The two valves can be 

 readily distinguished by the presence or absence of this notch, and it 

 will be convenient to use preferably the terms "lower" "and upper." Six 

 dimensions were taken on each valve ; namely, first the dorso-ventral 

 diameter, perpendicular to the hinge line at the beak ; second and third, 

 the antero-posterior distance in front of and behind this line (the sum 

 of these, naturally, gives the total antero-posterior dimension) ; fourth 

 and fifth, the length of the anterior and of the posterior part of the 

 hinge, measuring from the beak (the sum gives the total hinge length) ; 

 and sixth, the partial dextro-siuistral diameter, or the greatest perpen- 

 dicular distance from the plane passing between the two valves to the 

 outside of the valve. The measurements were made with a special 

 instrument, which might be called a valvameter, constructed as follows : 

 At a point on the circumference of a circular planed steel base 11 mm. 

 thick and 170 mm. in diameter, there rises perpendicularly a scale, 

 graduated to millimetres. Upon this scale slides an arm, which is 

 parallel to the base and extends out over its centre and also bears a 

 vernier, by which distances on the scale may be read, from the base, 

 to tenths of a millimetre. Upon the base is pasted a piece of metric 

 quadruple-ruled paper so that one system of lines runs parallel to the 

 arm. The valve to be measured is placed flat on the basal plate with 

 its beak touching the vertical scale and with the hinge line strictly tan- 

 gential. The zero lines having been carefully adjusted at the time of 

 affixing the quadruple-ruled paper, one can read oiF at once the dorso- 

 ventral and the two parts of the antero-posterior diameter and the two 

 partial hinge lengths. By lowering the arm until it touches the highest 

 part of the shell, one can read off on the vertical scale the transverse 

 diameter of the valve (partial transverse diameter of the animal). 



