Given that en coquille refers not to the 2D shape but rather to the 3D shape of an arch, and assuming the archivolt is not actually scalloped, I'd look for something other than anything shell-related since I think one generally thinks of the 2D representation.

"En architecture, la coquille est la voûte en quart de sphère, parfois décorée de cannelures et formant la partie supérieure d'une niche en plein cintre."
https://www.meubliz.com/definition/coquille/

Given this definition, I assume that in your case, rather than having a vertical tympanum above the door (inset from the façade) and a series of archivolts progressively working out to the façade, the tympanum is curved both vertically (to meet the bottom archivolt) and horizontally (to carry the curvature of the successive archivolts deeper into the wall (see images).

This shape goes by two names: semi-cupular or quarter-spherical.

So, semi-cupular archivolt, quarter-spherical archivolt, or even simply concave archivolt, maybe.

On the other hand, if it is indeed somehow sculpted to resemble a shell, it could be a 'shell-headed archivolt', maybe, along the lines of a 'shell-headed niche' (see image), even if the archivolt is itself the head of the building component below:

"shell-headed Said of a decoration, generally concave in shape, that is often similar in appearance to the shell of a sea scallop; often found at the head of a building component in Spanish architecture and its derivatives."
Dictionary of Architecture & Construction, Ed. Cyril M. Harris, ISBN 0-07-135178-7, €96 from W.H.Smith, Paris in the 1990s

Of course, just as there are no Google hits for 'shell-shaped archivolt', as Johannes has pointed out, neither are there any for any of my proposals … except, that is, 'concave archivolt'. And where did we get 'concave'? The Latin cavus means 'hollow' and the Latin concha means 'shell (of snail)', which leaves my wondering if 'concave' does not actually mean 'hollow like a shell'.

It consists of a large portico with two pairs of twin Corinthian pilasters, which carry a concave archivolt in a semi-circular arch.
https://www.waymarking.com/waymarks/wm11HCV_Chapelle_de_lOra...

"ornamented lintel and cornice surmounted by ornate concave archivolt with festoons and foliate design housing peacock fanlight"
https://www.buildingsofireland.ie/buildings-search/building/...

" Composite engaged columns supporting entablature, with dentils and concave archivolt having moulded enlarged dropped keystone"
https://blackwatercastlecorkdotorg.wordpress.com/tag/buildin...

"Ionic-style engaged columns supporting frieze with swag decoration and dentillated cornice, concave archivolt having scroll keystone"
https://www.buildingsofireland.ie/buildings-search/building/...

"Round-head door opening with flush surround of ashlar Portland stone having concave archivolt"
https://www.buildingsofireland.ie/buildings-search/building/...

I have not found pictures of all the concave archivolts above, but those I have found (interestingly, with the exception of one in the Roero, Italy, they all seem to be in Ireland) are not exactly 'shell-shaped', the archivolt(s) extending only a short distance into the depth of the wall where the tympanum is actually a half-round window.

"concha 1. The semidome vaulting of an apse; also called a conch. 2. O, Spanish architecture and its derivatives, a decorative element in the form of the interior of a sea scallop; see shell-headed."
Dictionary of Architecture & Construction, Ed. Cyril M. Harris

"with its own little dome and conched apse"
http://www.churches-of-rome.info/CoR_Info/SNC045/SNinC.pdf

"Irish churches, in their general form, preserve very nearly that of the Roman basilica, but they never present the conched semi-circular absis at the east end"
https://theses.gla.ac.uk/79930/1/13905381.pdf

"The structural system of conched churches consisted of massive walls and a central area surmounted by a semicircular dome"

Conclusion: There does not appear to be a term for this particular building element, not that I have found, at least, so unless there is to be lengthy descriptive text, something has to be made up. A variation of Johannes' 'vault' suggestion would be as good as any IMHO: 'conch archivolt' or 'conched archivolt'.


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Note added at 1 day 21 hrs (2024-02-01 15:32:31 GMT)
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