GREGORY, PRESENT STATUS OF ORIGIN OF TETRAPODA 321 



is supported hy a stout cartilaginous framework and the contractilit}' of 

 the anterior pair of arches has led to their functioning in the seizing 

 and ingestion of the prey: secondly, the primary locomotive organs, 

 namely, the myomeres, have much increased in number and size, the head 

 becoming relatively small; thirdly, tlie paired sensory capsules (olfactory, 

 optic, auditory) and the primitive brain-trough are synthetized into a 

 chondrocranium, which very early aff^ords support for the oral arches, 

 while the more active life necessitates the strengthening of the primitive 

 endoskeleton, an end which is attained in this group by the deposition of 

 calcareous salts in the hyaline cartilage; fourthly, many accessory struc- 

 tures appear : such as median and paired fins and fin supports, fin-spines, 

 true teeth and minute rhombic scales. 



All these external improvements in the locomotive and food-getting 

 structures were no doubt matched by corresponding increase in the size 

 of the brain and probably by marked changes in the proportions and 

 placement of its principal parts. 



Such a transformation from pre-gnathostome to primitive gnathostome 

 conditions was a critical step in the history of the vertebrates and makes 

 the differences between Tetrapocla and true fish seem relatively unim- 

 portant. 



The elasmobranchs are excluded from direct ancestry to the Tetrapoda 

 by the fact that they too early overspecialized in some respects while 

 remaining on a low stage in others : first, the exoskeleton was formed too 

 largel}^ from the outer layers of the many-layered skin, the deeper, strati- 

 fied connective tissue remaining unossified; hence except in the Acathodii 

 we see an armature of thorny denticles or placoid tubercles rather than 

 of osseous plates and scales. And secondly, the endoskeleton, instead of 

 becoming osseous, became thoroughly calcified. 



The earliest elasmobranchs have the preorbital portion of the cranium 

 short, the small e^^es being almost terminal and not widely separated from 

 each other, as in most of the ostracoderms, antiarchs, arthrodires, earliest 

 ganoids and tetrapods. It is only in specialized types in all these groups 

 that the olfactory capsule or rostrum grows forward and the eyes are 

 displaced backward and become of large size. 



So far as known the elasmobranchs lack the median opening between 

 the orbits which is doubtfully homologized with the pineal opening in 

 ostracoderms, antiarchs, arthrodires and primitive ganoids. 



ACTIXOPTETIYGII 



The Dipnoi, Crossopterygii and Actinopterygii (collectively known as 

 Osteichthyes) share with the elasmobranchs on the one hand nnd with 



