The Encyclopedia

Christianity/Caesars
A number of Caesars (Julius, Augustus, Tiberius, Claudius, Nero) ruled Rome before Christianity, once a persecuted faith, became dominant in Rome.

catacombs/Rome

Jean-Jaques/Diogenes/Calvin/Socinius
Two philosophers: Jean-Jaques Rousseau and Diogenes. Two (three?) refomers of the Protestant faith: John Calvin (1509-1564), and either Faustus Socinus (1539-1604) or Laelius Socinus (1525-1562) (two members of the same family who worked on the reformation and founded the Socinianism doctrine).

John Huss/Luther/Descartes/Voltaire/Condorcet/Robespierre/Marat/Babeuf
John Huss (or Hus) (1370(?)-1415) reformer of the Church, predating the Reformation in the 16th century; he was executed as a heretic, which set off the Hussite wars in Bohemia. Martin Luther (1483-1546), who published his Ninety-Five Theses, and initiated the Protestant Reformation. Rene Descartes (1596-1650), philosopher ("cogito ergo sum"--"I think; therefore, I am" (Lat.)) and mathematician (primarily analytical geometry, Cartesian coordinates), who placed emphasis on rational thought. Voltaire (pen name of Francois Marie Arouet) (1694-1778), author and philosopher, imprisoned then exiled from France for much of his life for his criticism of the government and the church. Condorcet. Robespierre. Jean Paul Marat (1743-1793), French revolutionary, elected to the Convention Aug. 1792(?); stabbed to death in his bath. Francois Noel Babeuf (1760-1797), another French revolutionary; proposed common ownership of all property and land (see also).

Saint-Simon/Owen/Fourier
Various socialists. "Owen" probably refers to Robert Owen, but which one?

le troisieme dessous
(Fr.) The "third lower floor" mentioned at the beginning of the chapter.

Inferi
Those down below; the dead.

Ugolino
My best guess is Count Ugolino; in the thirteenth century, he was (along with his children) imprisoned in the Tower of Pisa for treason, to die of starvation. One of the damned souls in the ninth circle of Hell described in Dante Alighieri's Inferno; also depicted in Geoffrey Chaucer's "The Monk's Tale," one of the Canterbury Tales.

Lacenaire

Babeuf/Cartouche/Marat/Schinderhannes
Babeuf. Cartouche, the 18-century French Robin Hood (a French film titled "Cartouche" exists about him, made in 1963). Marat. Schinderhannes, the German robber-hero (a la Robin Hood) of the Rhine.

Hercules
Or Herackles. Mythological Greek hero, son of the king of the Greek gods, Zeus, and the mortal woman Alcemene. Reknowned for his great physical strength and for his completion of the twelve monumental labours imposed on him by the king Eurystheus.

Farnese Hercules
Sculpture of Hercules (see above) created 4th century B.C. by the Roman artist Glykon, based on an original by Lysippos (or one of his pupils). Glykon's statue is in the Farnese Palace at Rome; a copy is in the gardens of the Tuileries in Paris.

Arche-Marion
Thirteenth-century Parisian street which runs down to the River Seine through an arch (l'Arch-Marion). Became rue des Bourdonnais in 1852.

Marshal Brune/"Avignon in 1815"
Guillaume Marie Anne Brune (1763-1815), general and marshal of France under Napoleon. He was murdered by royalists in Avignon in 1815.

Saint-Mihiel
Small town in Lorraine, France, on the Meuse River.

Cain/Abel
Biblical: Book of Genesis. The sons of Adam and Eve. Cain, jealous that God accepted Abel's offering (a lamb) and not his (fruits of the earth), killed Abel and was afterwards sentenced by God to roam the land.

Proteus
In Greek mythology, Proteus is the herdsman of Poseidon's (or Neptune's, in Roman mythology) sea-calves, and also a prophet. He has the power to change himself into any form he wishes.

Vidocq
Eugène François Vidocq, 18th-century French detective, founder of the Surete (the French police organization). (Side note: today, there exists a crime-solving organization named after him.)

Coco-Latour

the Seine
The river that divides Paris into the Rive Gauche (Left (following the flow of the river) Bank) and Rive Droite (Right Bank).

the Salpetriere
Salpetriere

President of the Assizes/Lacenaire

Montmatre or Montrouge

Horace/"Ambubaiarum collegia, pharmacopolae, mendici, mimae"
Horace. The quote appears in his Sermonum Liber I. (Translation?)