A sea of glass, like crystal, calm and transparent, and may signify that the saints had passed a boisterous sea of troubles in this world, which is now changed into everlasting tranquillity.

Four living creatures, or animals. Alcazar (p. 364) takes notice of thirty different expositions of these animals. He understands the apostles, bishops, and preachers of the Christian faith: others, four of the chief Angels or celestial spirits. Several others expound them of the four evangelists: yet this was before St. John himself had written his gospel. (Witham)

The extensive sea of glass, here described transparent as crystal, represents what may be called the floor of heaven. Before the throne and round it stand four living creatures, of an extraordinary shape, which denote the four great prophets, Isaias, Jeremias, Ezechiel, and Daniel. Their bodies are described full of eyes, both before and behind, an emblem of their prophetic sight, that penetrates into all ages past, present, and to come. And their being also full of eyes within, indicates that their extensive knowledge arises from an interior divine inspiration. They have each six wings, in the same manner as the seraphim appeared to the prophet Isaias. (Chap. vi. 2.) Some have imagined these four symbolical animals to represent the four evangelists; but we think improperly, as St. John was still living and there present in person. The first animal is here said to resemble a lion, the king of beasts, because the prophet Isaias, represented by it, was descended of the royal race of David. The second animal resembles a calf, and represents the prophet Jeremias in his character of priest; the calf, which was the principal victim in Jewish sacrifices, being on that account the emblem of the priesthood. The third animal, exhibiting Ezechiel, has the countenance of a man; because God, in speaking to that prophet, always addresses him by the name of son of man. The fourth animal, denoting Daniel, resembles a flying eagle, on account of the sublime oracles of this prophet, who soars to the highest objects, and views the succession of all the great empires that were to rise up in the world to the end of time. Probably these four principal prophets are to be understood to represent all the prophets of the old law. (Walmesley)