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POPULAR SCIENCE MONTHLY 



Fig. 2. Restoration of the Skeleton of Theropleura. 



jaws the teeth are somewhat flattened, so that cutting edges are de- 

 veloped which would lacerate the prey in capture or before swallowing 

 and minimize the chances of escape. Jn Clepsy drops 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 sim- 

 pler forms in which there is no such fin or frill. It must be remem- 

 bered 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 pos- 

 terior 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 hold- 

 ing 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 



Fig. 3. Restoration of the Skeleton of Clcpsydrops. 



