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MR. T. W. BRIDGE OR THE OSTEOLOGY OF POLYODOR EOLIUM. 
Polyodon; the possession of jugal and quadrate ossicles, and well-developed osseous 
transverse processes, neural arches, neural spines and ribs. The massive growth of 
the cranial cartilage in Acipenser has obliterated the parietal foramina and the cavity 
of the prenasal rostrum, diminished the antero-posterior extent of the cranial cavity, 
and caused the abortion of the cranial prolongation of the notochord. The posterior 
median fontanelle of A cipenser has no existence in Polyodon, and in the former the 
fibrous tracts ossify as distinct splints, and not continuously, as in the latter. To these 
more or less important differences we may add that in Acipenser there is a well- 
developed opercular gill, a siphonal stomach, and a simple n on-cellular air-bladder ; 
whereas in Polyodon there is no opercular gill, the stomach is coecal, and the air-bladder 
cellular. With these, for the most part relatively unimportant exceptions, the agreement 
between the two genera, even in the minutiae of their skeletal anatomy, is very strik- 
ing. It will be noticed that, in so far as the Sturgeon recedes from the Polyodon type, 
it becomes more specialised, and approaches the higher Ganoids and the Teleostei. 
B. Comparison of Polyodon with the Elasmobranchii. 
Not the least interesting feature in the osteology of Polyodon is its possession of a 
remarkable combination of characters, usually regarded as specially distinctive of the 
Plagiostome Elasmobranclis. Of these, we may mention the unossified chondrocranium ; 
the formation of the upper jaw by the union of the pterygo- quad rate bars in a median 
symphysis beneath the basis cranii ; the attachment of the quadrate cartilage to the 
hyomandibular by means of symplectic ligaments; the abortion of the proximal 
segment of the mandibular arch and its representation by a metapterygoid ligament 
which bounds the spiracle in front, supports a mandibular gill, and is attached behind 
and below to the hyomandibular ; the presence of a Facial canal ; the rudimentary 
cartilaginous ray attached to the hyomandibular, as in Acipenser, which it is possible 
may represent the hyoid rays of the Selachii ; and the parosteal operculum and inter- 
operculum, resembling in their branched and subdivided condition the cartilaginous 
rays which they have replaced. 
The presence of intercalated neural arches, the prolongation of a fleshy lobe into the 
upper division of the caudal fin, the abortive condition of the ribs, and the structure of 
the shoulder girdle and pectoral fins, are also evidences of a like affinity. Of the more 
numerous minor points to which the same significance must be attached, I may mention 
the absence of a basihyal as in the Pays, the non-segmentation of the epihyal from 
the ceratohyal, and the prolonged prenasal rostrum, with its probably coalesced first 
pair of labial cartilages. On the other hand the condition of the ventral fins and of 
the vertebral column is far more primitive than in the Selachii. 
The lateral position of the anterior and posterior nares, the structure of the pectoral, 
ventral, anal, and dorsal fins, as well as the segmentation and disposition of the inter- 
spinous cartilages and their relation to the more numerous fin rays, are to some extent 
