Mermaid on the ceiling of Hôtel du Voué, 13th century
Metz, Musées de la Cour d’Or.
One of the best known of all the chimerical creatures of human form, the Mermaid had a long armorial history, stretching back to the early Middle Ages, [...] Partly she comes to us from the misty beginnings of European folk-lore, and was known to the ancient Celts, the Teutonic tribes, the Vikings and the early English.[...] At times she was equated with the Sirens of ancient Greece, whose melodious songs lured mariners to destruction; at others with the Harpies, those fell snatchers of men’s souls. [...]
Bartholomew Anglicus, writing around 1230, tells us that
‘the Mermayden, hyghte Sirena, is a see beaste wonderly shape, and draweth shypmen to peryll by swetenes of songe’. He goes on to say: Phisiologus speaketh of Sirena and saythe it is a beaste of the see, wonderly shape as a mayde from the navelle upwards and a fyshe from the navell downewarde, and this wonderful beaste is gladde and merye in tempeste and sadde and hevye in fayre wether. With swetenes of songe this beaste maketh shypmen to slepe, and when she seeth that they ben aslepe she gooth into the shyppe and ravysheth which she may take with her, and bryn geth hym into a drye place, and maketh hym fyrste lye by her and do the dede of lechery, and if he wolle not or may not, then she sleeth hym and eatheth his fleshe”
Lawrens Andrewe, (writing about 1527) however, knew how to deal with this nautical hazard, and said that ‘the wise mariners stop their ears when they see her, for when she playeth on the water all they be in fear, and then they cast out an empty tun to let her play with it till they be past her.’ [...]
While the mermaid is normally, and correctly, shown with one tail, there is a variety which has two fish tails. [...] In German armory, she is commonly called a Melusine. 
A german source writes:
“Sirene sint merwunder gar wol gestimmet, sam Aristotiles spricht . die mügent ze däutsch merweip haizen, wan si habent oben von dem haupt unz an den nabel ainr frawen gestalt und habent ain edel groezen und gar ain graussam antlütz . si habent auch auf dem haupt gar langez hâr und hertz, sam daz pfärdes hâr ist ... daz nider tail and dem tier ist als daz nider tail ains adlarn sam Adelînus spricht, und hât das tier gar scharpf kraeuln an den füezen ... und hât ze letzt ainen swanz mit schüepeln als ain visch.” 
....And here the confusion starts as she is described as having the claws of an eagle and a tail with scales.
A mermaid is often used to demonstrate a relation with the sea, in particular she is a popular figurehead of sailing ships.
Figurehed of a mermaid, 18th century
Also, sometimes she is used as a supporter of heraldic arms, for example of the arms of Naples:
Achievement of the Kingdom of Naples, 1700 ca
- In relation with Sicily the mermaids, (actually melusines) are known from some capitals in the cloister of Monreale cathedral. They are also on the gloves of the coronation vestments of Henry VI and Frederick II. In the 17th century they were chosen as supporters of the arms of the Kingdom of Naples but apparently they disappeared in the time of the Spanish Succession wars.
De reason why there was thought to be a relation between Sicily and a mermaid may be the confusion between a siren (which is a bird with a human face) and a mermaid (which is a virgin with a fish tail). Sirens appear in the capital about the passing of Scylla and Charibdis (i.e. the Strait of Messina) in Homers’ Odyssee. Sicily was the place where Proserpina, the daughter of Ceres, was kidnapped by Pluto. At the moment Proserpina had been accompanied by sirens, the daughters of the river-god Achelous. Ovid continues: “But how did the daughters of Achelous come to have feathers and claws like birds, while retaining their human faces? Was it because these skilful singers were among Proserpine’s companions, when she was gathering the spring flowers? And after seeking her in vain the world over, they prayed that they might fly across the waves on beating wings, so that the seas, too, might know of their anxiety. The gods consented, and suddenly they saw their limbs covered with golden plumage. But in case those melodies that fell so sweetly on the ear should be silenced, if the maidens lost their tongues, and their rich gift of song be denied expression, they retained the features of young girls, and kept their human voices.” .
Merman on the Pergamon Altar, about 180 BC.
The male counterpart of the mermaid is the merman which is a man with a fish tail instead of legs. Sometimes he has a trident in his hand. He is probably a version of Triton, the son of Poseidon and Amphitrite who could stir up and calm down the waves with his tritons’ horn, which had the form of a shell (compare the conch in hindu- and buddhist symbolism which was the symbol of religious power).
When the mermaid forms a couple with the merman the merman is often depicted with a trident in his hand and the mermaid with a mirror.
Achievement of the city of Muiden (Noord Holland, Netherlands)
But not always:
Achievement of Zierikzee (Zeeland, Netherlands) 17th century
The melusine, thought to be a woman with a serpents’ tail, was a special emblem of the House of Lusignan.
Melusine carrying the arms of Luxemburg/Bohemia and Lusignan/Cyprus
(From: Estienne de Lusignan: La Généalogie des 67 très illustres maisons, partie de France, partie étrang`res, issues de Mérovée. Paris, 1586)
The legend of Melusine from whom the Lusignans traced their descent was introduced at the beginning of the 14th century in a romance of chivalry of Jean d’Arras who, amongst others, was inspired by the Eneide. The legend goes as follows: When Elinas, king of Albania and his wife queen Pressine quarreled, their three daughters Palatine, Melia and Melusine decided to punish Elinas by confining him on a high and far away mountain in England where he could not escape because of a strong spell. Queen Pressine, missing a husband whom she had not wanted to be punished at all, took te following measures: Palatine was condemned to guard a sparrow hawk on a mountain in Armenia; Melia had to look after the treasures of king Elinas and Melusine had to transform herself every saturday in a fairy with a serpents’ tail. If her future husband ever discovered this mutation she would be deprived of matrimonial happines. Well then, one night dancing with other fairies, naked and with her hair hung down in a forest in Poitou, near the Fountain of Thirst, she was surprised by the son of the count of Forez who just had killed his uncle, the count of Poitiers. His mind put at rest by the view of the fairy the young man married her and settled with her in the castle of Lusignan built in fifteen days thanks to the supernatural powers of Melusine. The couple begot ten children who all knew a marvellous destiny: one married the daughter of the king of Cyprus, an other a daughter of the king of Bohemia and so on. But one saturday Melusine was surpised by her husband when she took a bath in a tub of green marble. The spell was broken and the fairy, transformed into a winged serpent, flew away. Nobody ever saw her again even when she was often heard later, roaming at midnight in several castles belonging to her royal descendants. 
...And here the Melusine does not have two tails but a serpents’ tail.
The arms and emblem of Warsaw is a mermaid with a shield but she developed from the picture of a dragon, also carrying a shield.
Very well known is the Little Mermaid in Kopenhagen, a statuette of a character in a fairy tale of H.C. Andersen
© Hubert de Vries