This manuscript, which contains a Tractatus de creatione mundi (Treatise on the World's Creation) from the Book of Genesis followed by a narration of the Passion of Christ (folios 99r–128v), is one of the most significant examples of late-13th-century Sienese illumination. The pictures, partly watercolor drawings and partly proper illuminations, were made by an extremely sophisticated Sienese artist who was heavily influenced by Transalpine miniaturists and active from around 1290 through the next decade. The illustrations, sketched by a fast, concise hand, stand out for their strikingly smooth style, unusual in the Sienese production of the time and a quality matched by the spontaneity of the narration and an uncommonly flowing hand. The landscape details make remarkable use of spatial illusionism, a sign of the artist’s awareness of the innovations by the Sienese painter Duccio di Buoninsegna (circa 1255–circa 1319). Scholars have put forward different theories about the identity of the artist, known as the Master of the Tractatus de Creatione Mundi, who created the series of illuminations illustrating episodes from the Creation and the lives of Adam and Eve. Art historian Luciano Bellosi suggested that they are by Guido di Graziano, creator of the 1280 Biccherna tablet, now in the Siena State Archives. Bellosi attributes to Guido a considerable number of works, including the dossal of Saint Peter in the Siena Pinacoteca Nazionale, which are stylistically very consistent with the illustrations in this manuscript. Ada Labriola, on the other hand, argues that the anonymous miniaturist was somewhat younger than Guido and probably trained in his workshop. She bases this conclusion on his more modern narrative style and the fact that the artist clearly was aware of innovations by Duccio and the Florentine painter Cimabue (circa 1240–1302). Labriola also recognizes the hand of this miniaturist as being distinct from the very similar one of the creator of a Crucifixion with Virgin and St. John and of an illuminated initial (folios 99r-v) decorating the Passio Iesu Christi composita ex quattuor evangelistis (Maestro of the Duecento of the Dominican Legendary). The manuscript is bound in a composite codex that gathers together five manuscripts of different ages (dating from the end of the 13th century to circa 1521) and provenance, and which are also dissimilar in layout, graphic style, and format.