Santa Maria in Macello Martyrum was a mediaeval church, latterly conventual, that used to stand on a crossroads diagonally opposite the Colonnacce (the remains of the Forum of Nerva ) in what was the Via Alessandrina. Picture of the church on Wikimedia Commons. 
It was dedicated to the Blessed Virgin Mary under her title of Our Lady of the Angels.
It originated in early mediaval times, but there is some difficulty in indentifying it with churches in the locality mentioned in contemporary catalogues. We have Santa Maria de Taurello, Santa Maria de Arcu Aureo and Santa Maria de Archa Noe as suggestions; it is thought that Santa Maria in Foro, Santa Maria in Guinizo and Santi Maria e Marcello were elsewhere. The earliest possible reference seems to be to a Santa Maria de Arcu in 1091.
In 1145, Pope Eugene III brought some alleged relics of St Mark from Castello di Giuliano into the city, and enshrined them here. The church was then called San Marco in Macello Martyrum, after a mediaeval market established nearby known as the Macellum Torrechianum. A macellum was an ancient Roman indoor market.
In 1517, Pope Leo X gave the church into the care of the guild of weavers -Confraternita dei Tessitori. They re-dedicated it to their patron saint, St Agatha, hence it was called Sant'Agata dei Tessitori. They restored it at some stage, and provided a plain Baroque façade.
However, in 1784 Pope Pius VI transferred it into the care of the Scalzetti. This small order of friars also had the church of Santa Maria delle Grazie a Porta Angelica, and they established a little convent here which became their general headquarters. The church was re-dedicated again, to St Mary of the Angels (Santa Maria degli Angeli). There was another restoration at the end of the 19th century.
The church was demolished in 1931 on the orders of Mussolini in order to make way for the Via dei Fori Imperiali. Two altars were rescued, and re-dedicated in the side chapels at Santa Teresa del Bambin Gesù in Panfilo.
The entrance was on a lost street called Via Croce Bianca, running to the west of the Via Alessandrina. The right hand wall of the church was on the latter.
To locate the site, imagine a lost street called Strada di Torre dei Conti which was a continuation of Via della Madonna dei Monti. This ran in front of the Colonnacce now on the Via dei Fori Imperiali, to form a crossroads with the Via Alessandrina and the Via Croce Bianca. The first-named was narrower than it is now, with the north-east walkway behind the line of the old frontage there. The church was diagonally opposite the Colonnacce on the former crossroads, in the present roadway where the pedestrian crossing is.
This small church had an orientation roughly south to north, and had a rectangular plan. The nave had three bays separated by pilasters supporting the vault, and the presbyterium was a narrower rectangular apse with a triumphal arch. There were four side chapels, two on each side.
There was a simple façade with a triangular pediment, and two pairs of pilasters without capitals flanking the doorway. These were continuous with a blank frieze below the pediment. The door was topped by a floating entablature, and there was a big tondo above this which had a fresco of the Madonna and Child. There was an oculus in the tympanum of the pediment, and a little bust under an archivolt in between each pair of pilasters, near the top.
A 18th century Vasi engraving shows the relationship of the church to the Colonnacce; see the "Romeartlover" web-page below.
One of the side chapels on the right had a crucifix in painted wood, and one on the left had an altarpiece of the 17th century showing St Anthony of Padua. The apse had a fresco of the 18th century, restored at the end of the 19th.