OCR Text |
Show 120 MR. H. G. P. SPURRELL ON THE [Feb. 6, The mandible of the snake articulates with the quadrate; the quadrate with the squamosal ; and the squamosal with the parietal (text-fig. 44). W h e n the snake wants either to seize its prey or to strike it with the poison-fangs in its maxilla?, it requires a wide gape. To get this, the movement is made at the quadrato-mandibular joint, which can be placed on a level with the teeth. The jaw-principle is then that of type 1. Having seized its prey, the snake, to swallow it, has to advance alternately the teeth in the movable maxilla and those in the mandible on either side. To move the teeth parallel with one another, the movement has to be made from the quadrato-squamosal joint, and so raised above the level of the teeth. The jaw-principle is then that of type 2. The parieto-squamosal joint allows the level of the quadrato-squamosal joint to be slightly lowered and brought forward; that is to say, freer antero-posterior and lateral movements to be made, and the passage between the quadrates to be slightly widened (text-fig. 44). Text-fig. 44. C Diagram of Snake's jaws. A. Squamosal. C. Mandible. B. Quadrate. D. Maxilla. The mandibles on the two sides are, of course, independent anteriorly. In most of the reptiles the jaws are of type 1 and are provided with long, sharply pointed and often recurved teeth to prevent the prey from slipping out of the mouth when seized*. These characters of the teeth are particularly well marked in a beast with a short muzzle, e. g. the Ceratosaurus. Long teeth are for obvious reasons less necessary in a beast with a long muzzle like the Gavial. The wavy line of the jaws in short-nosed Crocodiles is another device by which the slippings of prey are to be avoided. In the Iguanodon, however, the " teeth are not infrequently found worn down at the crown, like the molar teeth of the herbivorous mammalia at the present day" (British Museum Catalogue). " The worn down crowns form cutting, and at the * I have seen this accident happen. I gave an Ocellated Lizard a large slug which was very slimy and must have been as tough as india-rubber. The lizard picked it up and tried to bite it in half as though it were an earthworm, with the result that the slug shot out of its mouth to the distance of some inches. |