It is not without reason that the pilgrims returning from the threshold of Saint James bear shells
Although the scallop shell was adopted as an emblem by pilgrims to several shrines such as Mont Saint Michel, it was to become uniquely associated with the pilgrimage to Santiago de Compostela. The Liber Sancti Iacobi declares that “It is not without reason that the pilgrims returning from the threshold of Saint James bear shells”.
They were plentiful on the coast of Galicia where, presumably, pilgrims originally continued as far as the western sea shore as the final act of their journey to collect their emblem.
The Pilgrim’s Guide tells us that they were sold at the entrance to the cathedral by the middle of the twelfth century and by the year 1200 the selling of the scallop shell was regulated so that we know that one hundred scallop shell vendors were licensed at Santiago.
Archeological evidence for the association of the scallop shell with the Compostelan pilgrimage exists in the form of a shell which was discovered inside a tomb along the northern nave of the cathedral of Santiago.Due to its location it is dated no later than 1120. The first written evidence for the scallop shell as an emblem of the pilgrimage is in one of the miracles included in Book Two of the Liber Sancti Iacobi which is dated 1106. Thus we can be certain that the scallop was used as a symbol of the pilgrimage to Santiago de Compostela at the very start of the twelfth century and it seems plausible to conclude that a tradition had been established for some time before then.
In Antiquity depictions of the scallop shell in funerary imagery were intended to denote the heavenly afterlife and its use was appropriated into the symbolism of the early Church. The interior of the aedicule of Constantine’s church of the Holy Sepulchre at Jerusalem featured the form of a scallop shell carved into the niche.
It has also been noted that from prehistoric times, shells were placed around the bodies of the dead as part of funerary rites.
In Spain there seems to have been a particular association between the scallop shell and Christian imagery, going back to the Visigothic period. Examples are to be found at Merida and the church of San Tolme at Toledo and from the original church at Córdoba.
These replicate very closely the image on the silver and gold repoussé cover of the sixth century Sion Gospel. The similarity extends to the flanking palm trees which refer to Paradise. A notable difference however is the substitution of the Cross of the Sion Gospel with a Chrismon on the version found at Merida and Toledo.
The Chrismon is a particularly Hispanic image and there is a suggestion that it may have been especially important to the Visigoths with its reference to the Book of Revelation, a text which took on a more prominent function in the Visigothic church.
The Sion Gospel is considered to be the work of Byzantine craftsmen at Constantinople and to be dated to the 570’s. It is known that the Visigoths looked to Byzantium for cultural influences, unsurprising when one considers that they shared the Hispanic peninsula for a time in the sixth century after the Emperor Justinian conquered the Roman province of Baetica.
The Visigothic images of the scallop shell repeated the Antique representation which always portrayed the scallop as a concave form, reflecting its most common placement in a niche. The Compostelan scallop shell is distinctive however, in that it is always presented on its convex side and its orientation reversed, so that the narrow end of the shell is at the top. Now ressembling a shield rather than a receptacle this may simply be a matter of the change in usage from a sculptural architectural element into an emblematic form worn on clothing especially the scarcella, the purse or bag which became of one the attributes in depictions of Compostelan pilgrims.
The seashore to which Compostelan pilgrims directed themselves to collect their maritime pilgrim’s badge was not any seashore. It was in fact the 
point of land known as Finisterra, the end of the earth and one may speculate further as to what significance this may hold. This was the edge of the known world beyond which lay the western horizon, whose mythological connotations include ideas of the Land of the Dead and the resurrection symbolism of the sun’s decline and subsequent rising.
To this day, the coastline remains known as the Costa da Muerte and the numerous Celtic remnants still found in Galicia have given rise to suggestions that there was at the headland a cultic site, the Ara de Solis.
The proposition that rituals involving a ceremonial journey of the dead were performed there, prompts the question of whether the pilgrimage to Compostela was an adaption of an earlier cult associated with the passage to the afterlife, of which the scallop shell as a vessel of transportation over the sea to a Paradisaical realm, remains an evocative vestige. This may be the source of one of the Apostle of Compostela’s legendary miracles, when a drowned knight was brought up out of the sea by the saint, covered in scallop shells and restored to life.
Sources: The Pilgrims Guide to Santiago de Compostela William Melczer Italica Press 1993 The Miracles of Saint James Ed Thomas F Coffey, Linda Kay Davidson, Maryjane Dunn Italica Press 1996 Compostela and Europe: The Story of Diego Gelmirez Xunta de Galicia Skira Editore 2010 p.308 Victoriano Nodar Images and Symbols, Mircea Eliade 1961