Linda

Different authors use different terms, the terms I have used in my work on
the bivalvia is for the height, the greatest distance between the dorsal and
ventral margins; length the greatest distance between the anterior and
posterior margins and width the greatest distance between the external
(medial) surfaces of paired valves or in the case of a single valve the
greatest distance between the external surface and the shell margins. In the
case of Spondylus, Pectinidae etc the height is the greatest distance
between the umbones (ears) and the ventral margin and in the Spondylus the
width is the greatest distance between medial external surfaces (paired
valves)excluding spining. To figure out the correct way to measure the
margins is to draw a line centrally between the muscle scars of the valve to
the anterior and posterior margins and take a 90 degree angle from the
centre of this line to the dorsal margin, this gives the accepted way to
take these measurements.

Best wishes Kev