I took the text from the article "Ancient Scientific Basis of the â€œGreat Serpentâ€ from Historical Evidence", Richard B. Stothers, Isis. Volume 95, Issue 2, Page 220â€“238, Jun 2004. (link)
The source for the quote is given as:
"Orosius, Against the Pagans 4.8, trans. by Irving W. Raymond, Seven Books of History Against the Pagans: The Apology of Paulus Orosius (New York: Columbia Univ. Press, 1936), pp. 170â€“171. I have made a few changes in Raymond's translation; the phrase â€œthe feet of the ribsâ€ is italicized in light of the discussion below. See also the summary (Periocha) of Livy's Book 18. The eight sources that note the reptile's length as 120 feet are Valerius Maximus, Memorable Deeds and Sayings 1.8, ext. 19; Pliny the Elder, Natural History 8.37; Aulus Gellius, Attic Nights 7.3; Vibius Sequester, Geography, s.v. â€œBagradaâ€; Orosius, Against the Pagans 4.8; Nepotianus, Epitome 1.8, ext. 19; John of Damascus, Dragons 1; and Zonaras, Annals 8.13. The rounded number of 100 cubits (about 150 feet) is offered by Silius Italicus, Punica 6.153 (whose full version of the story is told in 6.140â€“293, 6.677â€“679). Those who simply describe the monster as huge are Seneca, Letters 82.24; Florus, Epitome 1.18.20; and Arnobius, Against the Pagans 7.46(43)."