io8 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



For the study of the skull of Necturus, complete skulls, preferably preserved 

 in fluid, and chondrocrania, from which the membrane bones have been removed, 

 should be at hand. 



i. General regions of the skull. The skull is partly bony, partly cartilagi- 

 nous. The bony part exists in the form of distinct areas or bones, separated from 



FIG. 38. Lower jaws of four vertebrates to show the reduction in the number of bones in the 

 course of evolution. A, lower jaw of an extinct amphibian, Trimcrorhachis, belonging to the Stcgo- 

 ccphala; inner surface above, outer surface below; note large number of membrane bones. B, lower 

 jaw of an extinct reptile, Labidosaurus, belonging to the Cotylosauria; inner surface above, outer surface 

 below; note reduction in the number of membrane bones and increased size of the dentary h and the 

 splenial i. C, lower jaw of a modern reptile, a lizard, Varanus, showing still farther reduction in the 

 number of bones; outer surface above, inner surface below. D, half of the lower jaw of man, seen 

 from the outer surface; it consists of but one bone, the dentary, all other bones having vanished. Mem- 

 brane bones blank; cartilage bones stippled, a, precoronoid; b, intercoronoid; c, postsplenial; d, coro- 

 noid; e, articular; /, angular; g, prearticular; h, dentary; i, splenial; j, supra-angular or surangular; 

 I, coronoid process; m, condyloid process; n, ramus; o, body; p, mental foramen. (A and B from 

 "Williston's Water Reptiles of the Past and Present, University of Chicago Press; C from Reynolds' The 

 Vertebrate Skeleton, courtesy of the Macmillan Company; D from a specimen loaned by the anatomy 

 department.) 



each other along wavy or jagged lines, the sutures. The membrane bones are 

 somewhat distinguishable from the cartilage bones by their more superficial 

 positions. The skull is divisible into a median portion, the skull proper, and 



