4 1 4 DINOSAURIA 



ment, recalling the Crocodilian condition, results in an increased 

 capacity of the dorsal portion of the body-cavity. Intervertebral 

 articulation of the ribs does not occur except sometimes in the 

 sacral region. Abdominal ribs are rare, but they occur in some 

 of the Theropoda, e.g. in Compsognatlms. 



The sternum seems to have been mainly cartilaginous, with 

 a pair of irregular, disc-shaped ossifications. How the coracoids 

 were attached is unknown ; they are small, generally with a 

 foramen, but the scapulae are always very strong and slant 

 backwards. Clavicles and interclavicles seem to be absent. 



The fore-limbs are as a rule powerful, although often much 

 shorter than the hind -limbs, which are then enormously 

 developed, and in many genera of two of the main groups show 

 a tendency towards a semi-erect gait. Some of the Dinosaurs, 

 e.g. Iguanodon and Brontozoum, were absolutely bipedal. Others 

 seem to have hopped like Kangaroos. In correlation with this more 

 or less erect mode of progression the iliac bones are very strong, 

 much elongated horizontally, and attached to more than three, 

 often to five or even more, vertebrae. The pubic bones show two 

 main types. Each consists either of a single strong shaft, which is 

 connected distally with its fellow ; or (Orthopoda) this main 

 shaft sends out, below its point of contact with the ischium, a 

 long process, the so-called post-pubis, which is directed down- 

 wards and backwards. In the latter case it runs parallel 

 and in close contact with the ischium. Such bifurcated pubic 

 bones never meet in the middle line. The ischia, on the other 

 hand, are always connected with each other, not so much by 

 fusion as by syndesmosis. 



The hind-limbs exhibit all stages from a simple, plantigrade 

 and five-toed state to a decidedly digitigrade, four, and even three- 

 toed arrangement. Many genera exhibit the tendency to form an 

 intertarsal joint, a feature elsewhere known in birds only, where 

 it is typical and universal. The astragalus sends up an ascending 

 process which tends to fuse with the anterior aspect of the distal 

 end of the tibia, and the calcaneum is sometimes more or less 

 firmly attached to the fibula. In Compsognathus even the distal 

 tarsalia have begun to fuse with the metatarsalia, so that this 

 reptile at least has a typical intertarsal joint. The femur is 

 remarkable for the frequent possession of a " fourth " trochanter 

 on the middle of the inner aspect of the shaft, undoubtedly 



