''"I'sJi!' ] P1M)C'EEDINGS OK 'I'llK NATIONAL MUSKUM. 453 



wiuds. These slopiii*;- faces of IIm; matrix represent a i>air nl" osseous 

 ]»lates, wliich jlescended on each side I'roni tiic; sheatli <•!' llir iiivtl<»u 

 and ehoi'da (U)isalis, for tlie hitter occupied this position in lh»' j^roove 

 aheady descriheti. iSnch a strneture would indi<;ate the presence of a 

 number of fixed vertebral elements, such as exists in the chinncras, the 

 rays, and the stur<;eoiis. The two-thirds of tiie inferior face of the 

 skull which lies in front of this groove is (;overed by a sin<;le thin plate, 

 which may be the paraspheiioid. Its posterior border reaches to the 

 anterior extremity of the roof-shaped descendin^^ plates already de- 

 scribed, and, joiniiifjf them by a rounded aii^le, turns downwards ami 

 outwards, the descending" portion slopinj^ forwards into the horizontal 

 l)ortion. Where it joins the descending" plates of the axis tlieri^ are 

 three j;;rooves on each side, which are separated by two ribs. At the 

 point of jumttion of the i)aiaspheiioid with the lateral al;e of the axis, 

 is situated what I suppose to be the foramen magnum. It is the direct 

 continuation of the groove already described, and, being tloored by the 

 jyarasphenoid, has a triangular section. There is no trace here of a 

 fossa for the chorda dorsalis, nor of an occipital condyle, nor is it prob- 

 able that either existed at this i)oint. The paraspheiioid is thin, and 

 there are no indications of teeth to be observed on it. 



For the opportunity »)f studying this specimen I am greatly indebted 

 to Prof. .1. ^V. 8i)encer, of the I'niversity of Missouri, and to the late 

 l)iesident of that institution, Prof. S. S. Laws, who lent it to me out of 

 their museum. 



I here describe the characters presented by another specimen of 

 Macropetalichthys which belongs to the geological museum of the State 

 ofOhio.and wlii(rh was kindly lent meby the director of the survey, Prof. 

 I»^(lward Orton. This specimen is broken transversely across the median 

 jtart of the iirea which includes the median ()rci])ital j)late, show ing 

 that tiie jtosterior part of that area is adistinct element separated Irom 

 it by suture. I call it therefore the median nuchal plate, and the two an- 

 ^iilarclcments on each side of the posterior region, which are also shown 

 to bi' distinct, 1 (;all the lateral nuchal elements. One of these is 

 wanting in the specimen, showing that its Junction with the median 

 element is by a smooth sijuamosal suture. The anterior face of the 

 nuchal mass has a vertical groove on the middle line which lits a cor- 

 responding keel of the cranium proper. The triangular foramen mag- 

 num issues at the inferior extremity of this keel; at the lateral extremity 

 of this occipitonuchal suture under the free lateral margin of the skull 

 is a fossa, one-half of which is in the cranium and one-half in tiie nuchal 

 element. Tiiis looks like an articular glenoid cavity, possibly for the 

 condyle of a mandible. It is bounded posteriorly by a transvers«^ crest, 

 posterior to which is the extensive longitudinal fossa beneath the free 

 border of the nuchai ))late. There is a small fossa on the middle line 

 20 millimeters in front of the occii)itonuchal suture, in the ])arasphe- 

 noid bone. The anterior i)art of the skull is better preserved than iu 



