I was reading a shell description and came across the followiing terminology: “Adult whorls etched by numerous fine prosocline growth lines.” That was a new one on me. So I turned to my dictionary and searched the web - no luck. So, when stumped, ask Harry Lee. Here's what he explained to me:

"Prosocline/opisthocline are terms applied to snail shells and describe the relationship of structures to shell's axis. Proso- means forward-slanted and opistho- means backward-slanted vs. a plane rotated along (not across) the shell's (columellar) axis - the idealized plane in which the labrum (outer lip) advances in a shell held apex upward. For example, Nerita peloronta has a prosocline labrum (outer lip), and the axial ribs of the Turbonilla in the photograph imbedded in the email to which I am herewith responding, although slightly sinuous, are overall opisthocline."

I replied with my interpretation of Harry's explanation and got it wrong. So, I tried again using an analogy more familiar to me and got it right. Just in case some of you may run into these terms and have difficulty with them as I have, maybe you'll fine this helpful.



Let’s assume these shells are aircraft fuselages with the apex the nose end. We’re looking down on the top of the aircraft and the black represents the plane running through the axis and wings (if they were there). The prosocline vs opisthocline directions would be the same as right or left yaw (a right or left turn in the black plane). If we yaw the white axis line to your right (as if making a turn to your right) it will align with the ribs on the Turbonilla , which would be opisthocline. If we yaw the white axis line to your left (as if making a left turn) it will align with the ribs on the Epitonium and be prosocline.

A simple way is that when you hold a spiral shell as in these photos, sculptural features like ribs that slant from shell's left to right are opisthocline and those that slant from shell's right to left are prosocline.

P. S. Ribs that align with the axis (white line) would be "orthocline."