Aristotle [ca. 350 BCE] (De animalibus, Book 2, 1.7; 6, 11.3-4): [Book 2, 1.7] The seal is like a maimed quadruped, for immediately beneath the scapula it has feet like hands, as are also those of the bear, for they are five-fingered, and each of the fingers has three joints, and a small claw: the hind feet are five-fingered, and each of the fingers has joints and claws like those upon the fore-feet; in shape they are very like the tail of a fish.[Book 6, 11.3] The seal is amphibious, for it does not inhale water, but breathes and sleeps. It produces its young on land, but near the shore, in the manner of animals with feet; but it lives the greater part of its time, and obtains its food in the sea, wherefore it is to be considered among aquatic animals. It is properly viviparous, and produces a living creature, and a chorion, and it brings forth the other membranes like a sheep. It produces one or two, never more than three young ones. It has also mammae, so that it suckles its young like quadrupeds. It produces its young like the human subject, at all seasons of the year, but especially with the earliest goats. [Book 6, 11.4] When the young are twelve days old, it leads them to the water several times in the day, in order to habituate them by degrees. It drags its hinder parts along, and does not walk, for it cannot erect itself upon its feet, but it contracts and draws itself together. It is fleshy and soft, and its bones are cartilaginous. It is difficult to kill the seal by violence, unless it is struck upon the temple, for its body is fleshy. It has a voice like an ox. - [Cresswell translation, 1887]

Pliny the Elder [1st century CE] (Natural History, Book 9, 15): The aquatic animals clad with hair are viviparous viviparous - for instance the sawfish, the whale and the seal. The last bears its young on land; it produces after-birth like cattle; in coupling it clings together as dogs do; it sometimes gives birth to more than two in a litter; it rears its young at the breast; it does not lead them down into the sea before the twelfth day, thereafter continually accustoming them to it. Seals are with difficulty killed unless the head is shattered. Of themselves they make a noise like lowing, whence their name 'sea-calves'; yet they are capable of training, and can be taught to salute the public with their voice and at the same time with bowing, and when called by name to reply with a harsh roar. No animal sleeps more heavily. The fins that they use in the sea also serve them on land as feet to crawl with. Their hides even when flayed from the body are said to retain a sense of the tides, and always to bristle when the tide is going out; and it is also said that the right fin possesses a soporific influence, and when placed under the head attracts sleep. - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.22, 6.28, 6.55): [Thomas describes the seal under the names helcus, koki and vitulis.] [Marine monsters 6.22] Helcus is a marine animal that is called a 'calf'. It has a hairy skin distinguished by black and white spots. And what is said of this animal is wonderful, that when it is dead and skinned, the fur on the skin, by a certain natural instinct, wherever it is, just as the sea is, so is the fur. For when the troubled sea rises up into waves, so too does the hair rise up. But if the sea has been pacified, the fur is spread out flat: thus the state of the sea is detected in senseless and dead matter. Pliny: It gives birth on land after the manner of cattle. It never gives birth to more than one pair, it brings up the offspring and suckles them from their mothers, and does not bring them to sea before twelve days. They are killed with difficulty unless the head is cut off. It has a moaning voice. No animal is more oppressed by sleep. Whence the right fin, which they use in the sea, are said to possess soporific power. [Marine monsters 6.28] A koki is a marine animal, as Aristotle says. But it lives on the land as well as in the sea. It does not breathe in the water. It sleeps on land near the sea, and there the greater part of its home is. There she gives birth to her offspring; but she gives birth to two or three. In childbirth it delivers like a human. On the twelfth day after birth, the children follow their mother to the sea, and many times during the day, and for this reason, to accustom the children to the waters of the sea. This animal has breasts, from which it feeds milk to its young. It has a cartilaginous body with bones that are less robust, so that sometimes it can hardly walk on it weak legs and support itself on its feet. But its walking is more due to the vigor of the spirit and the lightness of the body than to strength. It can hardly be killed except with hammers. Its voice is like the voice of a bull, and this especially when it is being killed. The nature of the female koki is like the nature of a woman, and in her belly is found a rod formed for intercourse. It has very sharp teeth, a small tail like a deer, kidneys like a cow. There is no blood in its belly; and this is the reason: the two veins which are under the kidneys branch out in them and do not reach as far as the belly; and therefore the belly of this animal lacks blood. But the other parts of the body have blood which never congeals. This animal has four feet and they have joints, five toes on each foot, and the shape of the foot is the shape of a fish's tail. [Marine monsters 6.55] Vytuli are fishes of the sea, as Pliny says; these breathe and sleep on the land. This animal has shaggy hair. It gives birth on land, and like cattle gives birth in a productive manner, with one offspring as is the case with dogs; it never gives birth to more than one pair. The child is raised by its mother. After the twelfth day she takes it into the sea, from which time she accustoms him to the great waters. They are killed with difficulty unless the head is cut off. Their voice is lowing sound, from which they receive the name of calf. No animal is more troubled by sleep. For this reason they say that their right fin has a somnambulist power, and that it promotes sleep. With the fins which they use in the sea, they also crawl on the ground with alternate feet. Their skins are said to retain the sense of the sea even when stripped from the body. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.110): Also in the sea is a beast lyke to the Calfe, and is therefore called the Sea calfe: and this beast calveth on the land, and gendreth as an Hound, and calveth never more than twaine, and he féedeth his whelpes with teates, and bringeth them not to the sea untill the eleventh day, and then he teacheth them to swim, and they are evill to slaye, except they be hit in the heads, and they lowe as a Calfe, and be therefore called Calves, and becke and make signes to men with voyce & with semblaunce with most discipline. No beast sléepeth faster than these, and with the fins that they use in the sea, they creepe on the lande, in stéede of feete, and have rough skins and hairie as calves have, and when the skinnes be falue off, they hold the kinde of the Sea, for the haire thereof ariseth when the sea floweth, his right fin hath a milde vertue, for it gendereth sléepe, if it be laid under the head. Huc usque Plin[ius] li. 8. ca. 7. - [Batman]