MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
701 
From what has been said, it is apparent that in all essential features the chondro- 
cranium of Acipenser resembles that of Polyodon. The massive growth of the cranial 
cartilage in the former has masked many features which are very evident in the latter. 
To this cause are due the obliteration of the fore part of the cranial cavity, which, 
consequently, appears to terminate behind the orbits where the brain also ends, though 
in reality it is continued as a very narrow canal, upwards and forwards, nearly as far 
as the internasal region, and the conversion of the rostrum into an almost solid mass of 
cartilage, traversed by anastomosing fat cavities. To the same cause is due also the 
obliteration of the passage by which it is probable the parietal depression primitively 
communicated with the cranial cavity. The growth of the cartilage round the margins 
of the parasphenoid, over the junction of the latter with the vomers, and round the 
tracks of the rostral divisions of the fifth nerve, which in consequence run in deep 
grooves bounded by high ridges, are other examples of the same tendency. The nasal 
sacs are shallow cups hewn out of the rostral cartilage instead of being appended to it 
by short peduncles, as in Polyodon. 
The large vacuity which interrupts the continuity of the roof of the cranio-spinal car- 
tilage, and which is situated mainly behind the vagus foramen, is absent in Polyodon, 
except, perhaps, in so far as the anterior termination of the supra- spinal canal may be 
its representative in the latter genus. In the Sturgeon the wing-like processes of the 
cranio-spinal cartilage run backwards and downwards from the postero-lateral angles 
of the cranium, which is the contrary direction to that taken by the lateral ridges in 
Polyodon. In both genera the unusual lengthening of the post-auditory and pre- 
orbital regions of the chonclrocranium as compared with the size of the brain, causes 
the Olfactory, the Pneumogastric, and the most anterior spinal nerves to run respec- 
tively forwards and backwards within the cranial cavity for a considerable distance 
before issuing through their respective foramina. A Facial canal, similar to but much 
longer and wider than that of Polyodon, exists also in Acipenser. 
The Visceral Arches. (Plate 57, figs. 8 and 9.) 
The two first visceral arches exhibit an interesting compromise between the pecu- 
liarities of the Elasmobranch and Teleostean types, for while the mandibular arch 
conforms mainly to the former type, the hyoid arch as distinctly follows the latter. 
As is the case with the hyostylic Selachians, the quadrate and the pterygoid process 
together form a continuous bar of cartilage — the so-called “ palato-quadrate bar ” 
(fig. 8, pg., Q.). This bar has no connexion with cranium, either directly or by 
ligament, but is ^suspended by its posterior extremity from the distal end of the 
symplectic, to which it is firmly attached by two slender ligaments. These ligaments 
pass from the rounded posterior margin of the quadrate cartilage backwards parallel 
to the anterior margin of the symplectic, and are attached, one to the proximal end 
of that cartilage, and the other to the contiguous extremity of the hyomandibular ; 
