ON THE OSTEOLOGY OF NYCTOSAURUS. 139 
Owen* says that ‘the ribs of Dimorphodon acquire a characteristic 
tenuity beyond the Sixth pair,” and Seeley mentions the fact that the 
posterior ribs of the pterodactyls are slender. In current restorations of 
the pterodactyls, the posterior ribs are often shown as long and strong 
bones, inclosing a large and deep abdomen, the outlet of which must 
have been through the narrow pelvis. It has been suspected that there 
are sternal ribs intervening between the vertebral ribs and the sternum, 
but there is no evidence of such in the present specimen. 
PEG TORALSGIRDEE AND EXTREMITY. 
So far as is definitely known, the pectoral girdle comprises the ster- 
num, coracoid and scapula only. No clavicles (unless the ossification 
hereinafter described is a clavicle), inter-clavicle, epicoracoids, or precora- 
coids, even, are known. Furthermore, the bones present are so strangely 
modified that they present very little resemblance to the same elements 
in other reptiles, agreeing rather better with those of birds, an agreement, 
however, that I believe to be homoplastic in nature. 
Sternum. Pl. XLII, Fig. 1. The sternum is a broad and thin bone, 
with a stout anterior projection, and a thin, spatulate xiphisternal process 
posteriorly. The presternal process is narrowed and rounded on the 
under surface, and may have projected somewhat ventrad. The saddle- 
shaped articular surfaces for the coracoids look dorsad and laterad. The 
anterior borders at the sides of the presternal projection are thickened, 
concave slightly in outline on the inner part, straight and more oblique 
on the outer ends. The lateral margins are thin, and are nearly parallel 
with the median axis of the bone. These borders have three emargina- 
tions, separated by four rounded projections, the anterior one longer and 
thicker than the others, which are thin and small. The posterior border 
is convex on either side, ending in a short and deep concavity at each 
side of the flat xiphisternal process. ‘This process is spatulate, with the 
distal end rounded or very slightly emarginate, and doubless gave attach- 
ment to the series of abdominal ribs, which lie in the specimen nearly 
in connection with it. The sternum is somewhat thickened in the middle 
anteriorly, or back of the base of the anterior process. On the sides and 
behind the bone is very thin, almost paper-like; this thinner part in the 
specimen shows numerous small crinkly fractures, doubtless caused by the 
flattening of the concave bone; similar crinkles are visible in the thin 
prepubic bones. The sternum lies nearly in its normal position in the 
skeleton. In falling backward it has been displaced a little obliquely to 
the long axis of the body, as have the dorsal vertebre, perhaps caused by 
* Paleontographica Soc., 1869, p. 69. 
