MR. T. W. BRIDGE OH THE OSTEOLOGY OF POLYODON FOLIUM. 
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Vertebral Column, and the Skeleton of the Fins. 
I do not propose to give a detailed description of tlie axial skeleton, as the desti- 
nation of my specimen would not admit of the requisite dissection. The large 
unconstricted and unsegmented notochord is surrounded by a relatively thick skele- 
togenous layer which supports the neural and haemal arches. The cartilaginous 
neural arches, though confluent near the cranium, are distinct in the central and 
posterior regions of the vertebral column. Above the spinal cord they coalesce, and 
dorsad of the true medullary canal a second or dorsal tube is formed between the 
slightly-divergent apices of the neuropophyses and the arms of the Y-shaped ossified 
neural spines, which terminates anteriorly on the upper surface of the cranio-spinal 
cartilage. A precisely similar canal is found in the Sturgeon and in Ceratodus, 
but not, so far as I am aware, in any Selachian or Teleostean. In the central 
n.s. Neural spine. 
d.c. Dorsal canal. 
sjp.c. Spinal canal. 
n. Notochord. 
slc.l. Skeletogenous layer. 
l.r. Lateral ridge. 
h.c. Haemal canal. 
n. Nemopophysis. 
s.r. Sub-notochordal rod. 
and posterior regions of the column the neural spines become double, one being 
attached to each neuropophysis, as is not unfrequently the case in certain Teleostei. 
Interposed between the true neural arches there is a series of small, triangular, 
intercalated cartilages. In front of each intercalated piece there is a small foramen 
for the exit of the anterior root of a spinal nerve, the posterior root passing outwards 
just over the apex of each cartilage, as shown in Plate 55, fig. 3. The haemal arches, 
which are not continuous with the neural arches, are attached along the ventral 
surface of the notochord ; each arch is incomplete above, along the medio-ventral line 
of the notochord, but below it completely encloses the dorsal aorta, the walls of which 
are co-extensive with those of the cartilaginous haemal canal. Each lateral half of each 
arch contributes to the formation of the continuous lateral ridge which alone repre- 
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