It would not be saying too much to assert that if the so-called sea-serpent were really a serpent, its entire length must be nearer 1000 than 100 feet. This, of course, is utterly incredible. We are, therefore, forced to the belief that the creature is not a serpent. If it were a long-necked reptile, with a concealed body much bulkier than the neck, the requirements of floatation would be satisfied; if to that body there were attached powerful paddles, the requirements of propulsion would be satisfied. The theory, then, suggested, first by Mr. Newman, later but independently by Mr. Stirling, and advocated since by several naturalists of repute, is simply that the so-called sea-serpent is a modern representative of the long-necked plesiosaurian reptile to which has been given the name of the _enaliosaurus_. Creatures of this kind prevailed in that era when what is called the lias was formed, a fossiliferous stratum belonging to the secondary or mesozoic rocks. They are not found in the later or tertiary rocks, and thereon an argument might be deduced against their possible existence in the present, or post-tertiary, period; but, as will presently be shown, this argument is far from being conclusive. The enaliosaurian reptiles were “extraordinary,” says Lyell, “for their number, size, and structure.” Like the ichthyosauri, or fish-lizards, the enaliosauri (or serpent-turtles, as they might almost be called) were carnivorous, their skeletons often enclosing the fossilized remains of half-digested fishes. They had extremely long necks, with heads very small compared with the body. They are supposed to have lived chiefly in narrow seas and estuaries, and to have breathed air like the modern whales and other aquatic mammals. Some of them were of formidable dimensions, though none of the skeletons yet discovered indicate a length of more than 35 feet. It is not, however, at all likely that the few skeletons known indicate the full size attained by these creatures. Probably, indeed, we have the remains of only a few out of many species, and some species existing in the mesozoic period may have as largely exceeded those whose skeletons have been found, as the boa-constrictor exceeds the common ringed snake. It is also altogether probable that in the struggle for existence during which the enaliosaurian reptiles have become _almost_ extinct (according to the hypothesis we are considering), none but the largest and strongest had any chance, in which case the present representatives of the family would largely exceed in bulk their progenitors of the mesozoic period.