OP THE LARTAL FOLTPTEEUS, SI 9 



slightly less than the width of the vessel, so that it can rotate freely on the bottona of 

 the vessel. A second and similar disc of glass has four segments cut off it in such a 

 way that when placed together and cemented to the first disc, so that their circum- 

 ferences coincide, there is formed a trough which will exactly carry the cube of 

 glass plates. 



If they fit properly the plates must stand vertically, and the cube of plates can be 

 rotated on the turn-table so that the reconstructed image can be seen from any aspect 

 by an observer always looking normally at the surface of the fluid, i. e. the side of the 

 square vessel. 



No image, of course, can be seen when the cube is so turned that the observer looks, 

 between the surfaces of the plates, but with a very slight turning from this position 

 the image at once comes into view. 



If it is desired to reconstruct only a small series of sections, the remainder of the 

 cube can conveniently be made up of squares of thick plate-glass (see text-fig. 2) in 

 place of thin mat- surface squares. 



III. — The Skeleton. 



The Chondrocranium. — The cranial anatomy of the adult Polypferics has been made 

 known by Traquair S wliile that of the half-grown Polypterus was described by 

 Pollard 2. 



In this small larva, measuring 30 mm., there is as yet but slight traces of ossification, 

 and that only in the membrane-bones of the head. The primordial cartilaginous 

 cranium may therefore be said to b© at the height of its development. 



The auditory capsule, though fused with the occipital and parietal region of the 

 cranium anteriorly, posteriorly is separated from the exoccipital region by a very 

 large foramen, giving exit to the xth and xith cranial nerves (PI. XXXIII. fig. 4, 

 X., XL). From this foramen a fissure extends between the auditory capsule and 

 the supraoccipital region, and likewise a fissure between the auditory capsule and the 

 basioccipital region. The exoccipital cartilage, though fused dorsally with the supra- 

 and veutrally with the basioccipital regions, is yet easy to recognize as a neural arch, 

 which is becoming fused with the cranium (PI. XXXIII. fig. 4, Ik.occ). The base 

 of this neural arch has the same form and position as the base of the first true neural 

 arch of the vertebral column, in front of which the first spinal nerve has exit 

 (PI. XXXIII. fig. 4, Sp.i.). 



The basioccipital region does not completely envelop the anterior end of the 

 notochord, but is composed of two halves (the parachordal cartilages) abutting on 

 the two sides of the front end of this structure (PL XXXIII. fig. 2). Posteriorly 



' Journ. Anat. & Phys. vol. v. (1871). 

 ' Zool. Jahrb. Anat. u. Out. Bd. v. (1892). 



