ME. J. W. DAVIS OjST chokdeosteus acipenseeoides. 609 



The orbit is oval, 1-2 inch from back to front, and 0*75 inch in 

 height. Its upper margin is bounded by the postfrontal (sphenotic) 

 bone, to the lower anterior extremity of which there is attached a 

 long and narrow bone (100), which bounds the orbit anteriorly and 

 separates it from the nasal orifice. The anteorbital region of the head 

 is, for the most part, lost. Sufficient indication of the arrangement 

 of the bones composing that part is left to show that the nostril was 

 large, and was, in its turn, surrounded by osseous structures forming 

 the anterior termination of the snout ; unfortunately the extent to 

 which the latter projected cannot be ascertained, but a comparison 

 of the bones composing the skull of the fossil with those of recent 

 species appears to indicate a short rather than a long snout. The 

 infraorbital margin is occupied by a bone, or series of bones, it is 

 not easy to determine which, extending backwards from the lower 

 part of the one already mentioned as dividing the orbit from the 

 nasal cavity beneath the orbit, forming a semicircular ring, and 

 joining the under surface of the cranium at the point where the 

 postfrontal is attached to the squamosal or mastoid. 



The base of the skull is formed by bones more completely ossified 

 than in the existing species of Sturgeons, and forming the roof of the 

 branchial, as well as the buccal cavity. The posterior portion of the 

 basisphenoid (6) extends posteriorly considerably beyond the point 

 of attachment of the suprascapula (post-temporal). It is broadly 

 expanded, thin, and fibrous in structure. Its extension forwards is 

 hidden for some distance by the epi tympanic (hyomandibular), 

 beyond which it can be again traced, extending beneath the orbit 

 and the nasal cavity. Lateral expansions of the bone appear 

 beneath the suture joining the mastoid with the frontal, and, in 

 all probability, represent the alisphenoid (prooticum) ; while the 

 extension further forward may represent the presphenoid or vomerine 

 portion of the roof of the mouth. 



The posterior margin of the head is formed by the scapular arch, 

 which extends in the form of a semicircle from the occipital region 

 of the skull, giving attachment to the pectoral fin, and extending 

 as far forward as the anterior margin of the operculum. The 

 upper portion of the arch is formed by the suprascapular (post- 

 temporal, 46), extending from the base of the skull 2*5 inches ; it 

 is long and narrow, thick posteriorly, and convex in outline ; 

 anteriorly, the bone is thinner and slightly expanded towards its 

 lower extremity. Attached to the suprascapula is the second bone 

 of the series, the scapular (supraclavicula, 47) ; it is 3*0 inches in 

 length, thick and strong, slightly over 1-0 inch in breadth in the 

 upper part, but becoming narrower towards its lower extremity. 

 The anterior portion of this element of the arch, like that of the 

 suprascapular, is comparatively thin and aliform. The coracoid 

 (clavicula, 48) is a large bone, extending beneath the scapula at 

 its upper extremity, and curving downwards anteriorly beneath the 

 opercular apparatus, by which it is, to a large extent, enveloped. 

 It is thick and rounded, affording a strong support to the large 

 pectoral fin. Its exact form is hidden by the overlapping operculum, 



