what exactly is the source mentioned here?
It is a palimpsest appended to the "Instituta Artium", a work written in the 3rd or 4th century AD.
Appendix Probi
The Appendix Probi ("Probus' Appendix") is a palimpsest appended to the "Instituta Artium", a work written in the 3rd or 4th century AD. 
what is geastrum quadrifidum? An inedible species of mushroom what is the origin of the name? Latin what does this species look like? The immature, unopened fruit body is roughly spherical to somewhat flattened or irregular in shape. what are its characteristics that can be observed through a microscope? Its basidia has a basal clamp connection and the spores in mass are dark brown when mature. where is the aforementioned language spoken? In the area around Rome and Italy. what are some types of that language? Old Latin, Classical Latin, Vulgar Latin and a few other. what are the ages that the second language was spoken in? Wilhelm Sigismund Teuffel defined the philological notion of it based on the metaphoric uses of the ancient myth as Ages of Man. what is unique about the vocabulary of the third language? It made some changes in spelling and case usage only. where did the name of the language come from? From the term "common speech" ("sermo vulgaris") were any changes made to this language at any point? Phonological changes can be seen in the late 3rd-century Appendix Probi
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Appendix Probi [SEP] Introduction
The Appendix Probi ("Probus' Appendix") is a palimpsest appended to the "Instituta Artium", a work written in the 3rd or 4th century AD. The text survives only in a carelessly transcribed water-damaged manuscript of the 7th or 8th century. In the past, it was attributed to Valerius Probus, but that is now considered wrong. The surviving manuscript is believed to have been transcribed at Bobbio Abbey, and it is currently kept at the Biblioteca Nazionale Vittorio Emanuele III. The "Appendix" lists common mistakes in the written Latin of the time. In those mistakes, tendencies in the grammar, spelling, and pronunciation of the contemporary vernacular can be observed that would become the various Romance languages.