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jaws the teeth are somewhat flattened, so that cutting edges are developed which would lacerate the prey in capture or before swallowing and minimize the chances of escape. In Clepsydrops the dentition is still more specialized; the incisors and canines are longer and the notch between them has developed into a toothless diastema; the skull is rather thin and high and the eyes are located far back in the skull. The spines of the vertebrae are very high and project from the back like the fin of a fish, in this matter contrasting strongly with the simpler forms in which there is no such fin or frill. It must be remembered that the spines are connected directly with the vertebral column and are not simply developed in the skin as is the case with the fishes. In the modern Basiliscus and Iguana there are frills on the back, but the strengthening spines are dermal like those of the fishes.

In Dimetrodon, the last of the series, these characters reached their culmination. The incisor and canine tusks have attained a relatively enormous length and strength, and projected from the jaws as much as three inches; the diastemal notch is larger and deeper and the posterior teeth of the jaws are recurved and have sharp serrate edges so they had all the cutting power of a Malay kris. Could a more effective arrangement be imagined for the cruel business of capturing and holding living prey despite its desperate struggles? The spines on the back developed to enormous length and in some forms tapered to the slenderness of a whip lash. The tail was short and the feet strong and with well-developed claws, all going to show that the animal was terrestrial in habit. The largest species of Dimetrodon reached a length of about eight feet and was easily the largest, strongest animal of its time. We can imagine this fiercely carnivorous form crouching in the bushes or tall grass on the side of some stream and making a