ON THE OSTEOLOGY OF NYCTOSAURUS. 153 
altogether reminding one very much of similar tendinous ossifications 
in the legs of many birds. 
A number of other slender, rib-like, more or less fragmentary bones 
are observed scattered about the slab near the body of the specimen, 
of which I am not at all certain, though I suspect that some of them 
are ventral ribs, perhaps attached during life to the extremity of the 
flat parasternal structures already described. One of these bones, lying 
by the lateral margin of the sacrum is about forty millimeters in length 
and is gently curved. It is flattened at one end, measuring nearly three 
millimeters in width, cylindrical at the other, and not more than one 
millimeter wide. Another, lying back of the pelvis, of about the same 
length is also slightly curved, one millimeter wide at one end and 
about three-tenths of a millimeter in width at the other end. There 
are also parts of several other bones which seem to be similar to these. 
SYSTEMATIC POSITION. 
Three specimens ascribed to /Vycfosaurus are now known — the type 
_ described briefly by Marsh, the one in the University of Kansas Museum 
described by me in 1892, and the present one. Marsh must have known 
more than one specimen, however, in which additional specimens he 
later recognized the essential generic characters. The three specimens 
differ materially in size, that described by Marsh being the largest, and 
the Kansas University specimen the smallest. In the smallest speci- 
men the coracoid and scapula are still distinguished by a separable 
suture. Furthermore, the separated arches of the dorsal vertebra, and 
the distinction of the elements of the atlas all indicate a young animal. 
On the other hand, the present specimen has all of its sutures obliter- 
ated, except that between the atlantal intercentrum and the axial inter- 
centrum, characters which may be ascribed to an adult condition. Among 
the characters given by Marsh, is the separated coracoid and scapula, 
but I suspect that this was derived from a smaller specimen than the 
one which he originally named P¢eranodon gracilis. I do not think 
that the absolute differences in size are sufficient to separate the speci- 
mens specifically, in that we know that similar differences are ascribed 
to other pterodactyls by some authors. In order, however, to make the 
comparative differences between these specimens clearer | have reduced 
them to a common standard, using the humerus as 100 in each. In addi- 
tion, I give like comparative measurements of certain species of P/erodac- 
tylus, derived from figures given by Zittel: 
