4 OSTEOLOGY OF PTERANODON. 



2 < in length. A slight break, probably occurring along the line of the original suture, 

 separates this from the basisphenoid bone, a long narrow ossification of triangular section 

 underlying the entire bony interorbital septum and terminating anteriorly in the basi- 

 pterygoid expansions. 



In the crushed specimen, broad and flat paroccipital processes extend outward and a 

 little downward from the occipital condyle. At their outer extremities they are com- 

 pletely fused with the hinder ends of the remarkable elongate bars formed by the 

 united quadrates and squamosals. The foramen magnum appears to have been a round 

 opening about 4mm } n diameter. Above this and separated from the paroccipital proc- 

 esses by narrow elliptical vacuities rises the triangular supraoccipital plate, bearing 

 throughout its entire length a thin but prominent median ridge. At this point the in- 

 ferior border of the great supraoccipital or sagittal crest originates. 



The skulls of the three species recognized in this memoir differ considerably in the 

 pitch of their supraoccipital plates. To express this more definitely, in P. longiceps 

 the supraoccipital plate rises less abruptly from the general axis of the basisphenoid 

 than in P. occidentalis, while in P. ingens the supraoccipital rises so little that its apex 

 is a trifle below the line made by prolonging the axis of the basisphenoid. The vary- 

 ing pitch of the supraoccipital plate may be regarded as a valid difference, for as the 

 three type skulls are crushed laterally the midline ossifications subtending these critical 

 angles have not been subjected to displacement. 



The extreme specialization of the American Cretaceous pterodactyls is well shown by 

 the form of the articular portion of the quadrate. Professor Marsh's figure of the type 

 did not sufficiently emphasize this structure, although he stated that the distal end of 

 the quadrate was one of the most characteristic parts of the skeleton. Dr. Plieninger 

 was probably the first to illustrate this peculiar bone correctly, but his monograph does 

 not throw light on its true significance and function. The distal ends of the quadrates, 

 instead of forming simple ginglymal joints with the mandibles, participate with them in 

 the formation of one of the most remarkable mechanical devices to be found in the 

 reptilian class. The articular surface of each quadrate bears a spiral groove, left handed 

 in the right quadrate and right handed in the left. The articular elements of the man- 

 dibles have a reciprocal form, their surfaces bearing stout threads exactly corresponding 

 with the quadratic grooves. The left quadrate of the type of P. longiceps, No. 1177, 

 with part of the jugal is shown in Plate V, figure 1. As the articular facet of the left 

 mandibular ramus of this skull is wanting, the same portion of another individual, No. 2578, 

 equal in size, has been used for illustration (Plate V, figure 2). Any possible difficulty 

 in the interpretation of these fragments will be removed by comparing them with the 

 quadrate and articular of No. 2476 (Plate V, figures 4, 5, and 6). So perfect is the 

 mutual adjustment of these parts that unless dislocation took place, which is not suppos- 

 able, opening the mouth must have caused considerable widening of the lower jaws 

 posteriorly. The quadrates being immovably fixed by the supporting bones of the 

 pterygo-palatine arch, an expansion of the posterior parts of the mandible was the only 

 way by which lateral motion caused by the spiral articulations could be taken up 

 mechanically. The fact is of especial interest in its bearing upon the prevailing suppo- 

 sition that the mandibular rami were joined by an immovable symphysis and their com- 

 ponent elements firmly anchylosed. 



In a preliminary description of this part (Am. Jour. Sci., July, 1903), the writer called 

 attention to the suspensorium of the Pelican, which is mechanically equivalent. Although 



