354 



WILLIAM PATTEN. 



There are in primitive fishes no head-muscles sufficiently 

 powerful to account for the development of the primordial 



Fig. 12. — The endocranium or cartilaginous sternum of Scorpion, seen 

 from the hsemal surface. The figure has been constructed entirely by 

 carefully plotting a complete series of transverse sections, only the 

 purely cartilaginous parts being drawn. — b. pi. Basal plate, forming a 

 thick floor to the central canal, m. c. Merochord, an hypophysis-like 

 segment of the spinal artery, n. cl. Neural canal or passage through 

 the occipital ring for the nerve-cords (compare section of this region 

 in Fig. 2, F). tr. Trabecula. z. Thickened portion of trabeculae 

 X. Thick bar of cartilage extending forwards from the neural surface 

 of the occipital ring, and forming an imperfect roof to the endocranium. 

 y. Membrane for support of merochord. 



cranium. As the cranial segments were very early immove- 

 ably united, the cranium could not have served for the attach- 

 ment of muscles moving the segments on each other for 

 purposes of locomotion as the vertebral column does in the 

 body. In fact, we can best account for the primordial cranium 

 of Vertebrates by supposing that it has been evolved independ- 

 ently of the spinal column, serving originally for the attach- 



