320 PROF. U. a. SKELEY ON SIMIIjITUJ>ES OS" 



mammals ; and no such condition is seen in Plesiosaurus. In the 

 dorsal region of birds there is a quadrate neural spine similar to 

 to that of Plesiosaurs, only shorter, and transverse processes 

 to the neural arch, which are horizontal laminae and not sub- 

 cylindrical processes ; the ribs in birds are articulated to the 

 side of the centrum on a concave facet, and only touch the under- 

 side of the transverse process with the posterior tubercle, while 

 in Plesiosaurus the rib is exclusively articulated with the trans- 

 verse process. The dorsal centrum in many birds has a promi- 

 nent hypapophysis similar to that of serpents ; but nothing of 

 the kind is seen in Plesiosaurus. Plesiosaurs have no sacrum ; 

 and if in birds the postfemoral vertebrae were put into the tail, 

 that region would be relatively as long as in Plesiosaurs. Ple- 

 siosaui's want the terminal caudal style of birds ; but the ordinary 

 caudal vertebrae of water-birds are not dissimilar to vertebrae of 

 Plesiosaurus, except in the massive or bifid neural spine ; and the 

 length and form of the centrum, as well as the absence of chevron 

 bones, make the tail -vertebrae of such birds as the Penguin or the 

 Swan more like the neck- than the tail-vertebrae of Plesiosaurus. 

 The caudal vertebrae of the Grannet only differ in size from vertebrae 

 of Plesiosaurs. The dorsal ribs of birds are always more compressed 

 than those of Plesiosaurs, and usually differ in having an epipleuron 

 anchylosed to the middle of the posterior margin of several. 



The pectoral and pelvic arches have nothing in common. 

 The difference in the pelvic region is presumably largely due to 

 the extension of the ilium along the whole length of the sacrum 

 in birds, while in Plesiosaurs it is a short conically tapering rod 

 which only meets one vertebra. The difference of the pectoral 

 arcli is presumably largely due to the great development of the 

 sternum in place of the system of abdominal ribs. If the sca- 

 pulae were to be drawn forward to meet in front of the coracoid 

 bones, the coracoids would themselves be drawn together ; and it 

 is not improbable that with such a plesiosaurian modification the 

 muscular attachments would move forward, and the sternum of a 

 bird would lose its continuous osseous character and large size. 



Neither humerus nor femur has much in common with Ple- 

 siosaurus. If the shaft of the humerus is compressed in the Pen- 

 guin, its distal end is not expanded, and its proximal articulation is 

 expanded too much. In the femur, if the hemispherical head is 

 directed inward, the trochanter external to it is too wide, while 

 the distal end is very dissimilar. 



