414 mk. k. w. hooley ox the [June 19 13, 



in a left humerus of Omiihostoma (Pteranodon). 1 In Seaphognathus, 

 Dhnorphodon, Dorygnqthus, and Pterodactylus it spreads out as a 

 wing from near the head of the bone. The two first-named have 

 the superior border concave, and the two last-named convex and the 

 wing deeper. In Rhamphorhynehus, Omiihostoma, and Nyctosaurus, 

 the deltoid crest is placed near the proximal end of the humerus ; 

 it rapidly constricts, and broadens at the tip into a deeply-obtuse 

 extremity. In Ornithodesmus it arises from below, and curves 

 spirally downwards. We have thus four well-marked types. The 

 species which come within these types are : — 



Type of Type of Type, of Type of 



Seaphognathus : Pterodactylus: Rhamphorhynehus; Ornithodesmus: 

 Sc. purdoni (?) Ft. antiquus. Rh. gemming i. O.latidens. 



"Dhnorphodon Ft. suevicus. Rh. muensteri. O. cluniculus (?) 



macronyx. Ft. longicollum. Rh. "kolceni. Humeri from the 



Ft. elegans (syn. Omithostoma in gens. Lower Chalk of 



Ft. pulchellus). JSfyctosaurus gracilis. Burham (Kent) 

 Ft. speetabilis. in the British 



Ft. medius. Museum (Natural 



Campylognathus zitteli. History). 



Dorygnalhus bathensis. 

 Ftenodracon brevirostris (?) 



The Seaphognathus type differs from the Ornithodesmus type : for 

 the reason that Ave behold in Seaphognathus the beginning, and in 

 Ornithodesmus the end, of a high specialization ; to the same cause 

 is probably due the fact that the deltoid crests of the humerus of 

 Campylognathus and Dorygnathus are closer to the Pterodactylus 

 than to the Rhamphorhynehus type. 



The bicipital crest of the early forms is prominently produced 

 outwards, and in the later very much reduced: for example, in the 

 Scaphognathoidea, Seaphognathus and Ornithodesmus; in the Kham- 

 phorhynchoidea, Rhamphorhynehus and Omithostoma. 



All these genera, so far as can be discerned, possess trochlear joints 

 at the distal articulation of the humerus, and nothing approaching 

 the complicated specialization of this joint in Ornithodesmus. 



The equal dimensions of the ulna and radius in the early 

 examples did not persist; the radius gradually became the smaller, 

 especially in the central region of the shaft. The proximal articu- 

 latory surface of the radius evolved from a general flat area to 

 two concavities divided by a ridge, for the trochlear surface of the 

 humerus. The distal articulation remained a simple pulley. 



The decussation of the ulna by the radius will probably prove to 

 be common to more than one genus : for instance, in some of the 

 Cambridge Greensand, Chalk, and American genera. 



The foramen which pierces the ischio-pubic plate of most species 

 does not occur in Ornithodesmus. 



1 G. R Eaton, ' Osteology of Pteranodon ' Mem. Conn. Acad. Arts & Sci. 

 vol. ii (1910) pi. xx, fig. 4. 



