Serapis
2 of the 7 encyclopedias on this shelf carry an entry for Serapis. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
the Roman name of a deity of Egyptian origin. The Greeks identified Serapis with their own Hades as god of the lower world. When the Greeks conquered Egypt they were led by political reasons to build a magnificent temple of Serapis, called from his name the Serapeum, at Alexandria, and united with the Egyptians in his worship. The following account of the Alexandrian temple and the worship of Serapis is taken from Milman's History of Christianity: The Serapion, at that time, appeared secure in the superstition which conected this inviolable sanctuary, and the honor of its god, with the rise and fall of the Nile, with the fertility and existence of Egypt, and, as Egypt was the granary of the East, the existence of Constantinople. The Pagans had little apprehension that the Serapion itself, before many years, would be leveled to the ground. The temple of Serapis, next to that of Jupiter in the Capitol, was the proudest monument of Pagan religious architecture. Like the more celebrated structures of the East, and that of Jerusalem in its glory, it comprehended within its precincts a vast mass of buildings, of which the temple itself formed the center. It was built on an artificial hill, in the old quarter of the city, called Rhacotis, to which the ascent was by a hundred steps. All the substructure was vaulted over; and in these dark chambers, which communicated with each other, were supposed to be carried on the most fearful and, to the Christian, abominable mysteries. All around the spacious level platform were the habitations of the priests, and of the ascetics dedicated to the worship of the god. Within these outworks of this city rather than temple was a square, surrounded on all sides with a magnificent portico. In the center arose the temple, on pillars of enormous magnitude and beautiful proportion. The work either of Alexander himself or of the first Ptolemy aspired to unite the colossal grandeur of Egyptian with the fine harmony of Grecian art. Egyptian and Greek met as worshipers of Serapis. The Serapis of Egypt was said to have been worshipped for ages at Sinope; he was transported from that city with great pomp and splendor, to be reincorporated, as it were, and reidentified with his ancient prototype. . . . The colossal statue of Serapis (in the Serapeum) embodied these various attributes. It filled the sanctuary; its outstretched and all-embracing arms touched the walls; the right the one, the left the other. It was said to have been the work of Sesostris; it was made of all the metals fused together--gold, silver, copper, iron, lead, and tin; it was inlaid with all kinds of precious stones; the whole was polished, and appeared of an azure color. The measure or bushel, the emblem of productiveness or plenty, crowned its head. By its side stood the symbolic three-headed animal, one the fore-part of a lion, one of a dog, one of a wolf. In this the Greek saw the type of their poetic Cerberus. The serpent, the symbol of eternity, wound round the whole, and returned resting its head on the hand of the god.
Collier's New Encyclopedia (1921)
or SARAPIS (also found as OSARAPIS), the Greek name of an Egyptian deity, introduced into Egypt in the time of Ptolemy I. or Soter, and really a combination of the Greek Hades and Egyptian Osiris. He was not an Egyptian, but the Greek deity, with some Egyptian characters superadded; and his temple was not admitted into the precincts of Egyptian cities, finding favor only in the Greek cities founded in Egypt. It is said that 42 temples were erected under the Ptolemies and Romans to this god in Egypt. His resemblance to Osiris consisted in his chthonic or infernal character, as judge of the dead and ruler of Hades. The god had a magnificent temple, the famed Serapeum at Alexandria, to which was attached the celebrated library; another at Memphis, in the vicinity of the cemetery of the mummies of the Apis, which was excavated by Mariette in 1850; and another temple at Canopus. It appears that he represented or was identified with the Hesiri Api, or Osorapis, the "Osirified" or "dead Apis," who was also invested with many of the attributes of Osiris. The worship of Serapis, introduced into Egypt by the Ptolemies, subsequently became greatly extended in Asia Minor; and his image, in alliance with that of Isis and other deities, appears on many of the coins of the imperial days of Rome. In A. D. 146 the worship of the god was introduced into the city of Rome by Antoninus Pius; but it was not long after abolished by the senate, on account of its licentious character. A celebrated temple of Serapis also existed at Puteoli, near Naples, and the remains of it are still seen. In Egypt itself the worship of the deity subsisted till the fall of paganism, the image at Alexandria continuing to be worshiped till destroyed, A. D. 398, by Theophilus, archbishop of that city.