292 



CAPT. D. M. S. WATSON ON SEYMOURIA, 



structure is that of a mosaic of separate details, some completely 

 amphibian, some completely reptilian, and very few, if any, 

 showing a passage leading from one to the other. In this fea- 

 ture our study of Seymouria lends support to the belief first 

 upheld by the Mendelians and now accepted and used by many 

 palaeontologists, that an individual animal consists of an aggre- 

 gate of separate characters which are to a large extent capable of 

 separate evolution, and may indeed proceed with their evolu- 

 tionary change at very different relative rates in different stocks, 

 although of course they react on one another so as to produce a 

 workable result. 



It thus appears desirable to have a tabular statement of the 

 characters in which Seymouria shows an advance on Temno- 

 spondyl structure, using so far as possible that of the Embolomeri 

 as standard, and pointing out where these advances are in 

 different directions from those which occur in the Rachitomous 

 Labyrinthodonts, the cousins of the Reptilia. 



List of characters in which Seymouria has advanced 

 above the Embolomeri : — 



1. The basioccipital is depressed and no longer presents a 



round concave condyle. Rachitomi parallel. 



2. There are tubera basisphenoidales. Rachitomi parallel. 



3. The exoccipitals exclude the basioccipital from the brain- 



cavity. Some Embolomeri and Rachitomi parallel. 

 [4. A hypoglossal foramen is present. ? If really absent in 

 Embolomeri, Rachitomi parallel.] 



5. The exoccipitals reach up to be overlapped by the occi- 



pital flanges of the dermo-supraoccipitals. Rachitomi 

 parallel. 



6. The supraoccipital is not ossified. Some Rachitomi 



parallel. 



7. The inner ear, though widely open to the brain-cavity, is 



distinctly separated by a rim of bone. Some Rachitomi 

 parallel. 



8. The fenestra ovalis is large and placed low clown so that 



its lower margin is formed by that of the basipterygoid 

 process. Rachitomi not parallel. 



9. The paroccipital is partly covered behind by an occipital 



flange from the tabular. Rachitomi parallel. 



10. The lachrymal extends from the orbit to the nostril. 



Rachitomi not parallel. 



11. The ductus naso-lachrymalis lies in the substance of the 



lachrymal. Rachitomi parallel (llicropholis). 



12. The choanaB lie near to the middle line. Rachitomi not 



parallel. 



13. The articulating surfaces of the zygapophyses are hori- 



zontally placed and the neural arches wide and swollen. 

 Rachitomi not parallel. 



