COMMENTS BY D. M. S. WATSON 983 



of the anterior and posterior semicircular canals, and the prootic, which 

 includes the anterior parts of the anterior vertical and horizontal semi- 

 circular canals and vestibule, sacculus, and lagena. 



The mammalian petrosal surrounds the wliole labyrinth. 



In Platypus there are two ossifications known agreeing in all features 

 with the prootic and paroccipital, and the supraoccipital does not include 

 any part of the labyrinth. The Monotreme petrosal is plainly homolo- 

 gous with that of man, which hence includes the reptilian prootic and 

 other things. The name petrosal can not, in consequence, be used for any 

 reptilian bone. 



Epiotic. — The Epiotic is said to be an element surrounding the upper 

 parts of the vertical semicircular canals. W. Iv. Parker claimed to have 

 seen it in Crocodilus and the chick. In the chick. Doctor Eidewood (B. 

 M. N", H.), who spent some weeks looking for it, assures me it does not 

 occur. I have never met anybody who could say that he had seen one. 

 I am thus doubtful of its actual existence. On the other hand, it is not 

 improbable that there really is an epiotic in Pteroplax, and I find that the 

 supraoccipital of Sphenodon begins as a paired double perichondral ossi- 

 fication, so that it might be regarded as a fused pair of epiotics and not 

 a supraoccipital formed in the tectum synoticum. 



Septoniaxilla. — This bone is a membrane ossification on the dorsal sur- 

 face of the paraseptal (Jacobson's) cartilage; it hence has nothing to do 

 with the Ethmoid, from which it is separate, even in Siplihonops, where 

 the "Ethmoid" is most extensively ossified. 



PostsplenialJ — The problem is, which of the two anterior infradenta- 

 ries of the Amphibia (Stegoceph) jaw becomes the splenial of reptiles. 

 The term infradentary has always been applied to all the elements of the 

 angular-splenial row in Osteolepids, and it seems undesirable to now re- 

 strict it to any individual member of that row. Professor Williston's 

 reasons for homologizing the "postsplenial" of Stegocephs with the 

 splenial of reptiles is that in advance of that bone there is a small fora- 

 men (the anterior mandibular), which seems to agree with the foramen 

 in the symphysis in advance of the splenial in reptiles. This argument 

 is good, but Professor Williston has overlooked the fact that a precisely 

 similar foramen in the symphysis in advance of this anterior element 

 does occur in Amphibia, I believe in all Stegocephalia. 



Hence there is just as much evidence in favor of the homology of the 

 anterior bone in Stegocephs as the posterior, and I prefer to retain Sple- 

 nial and Postsplenial. 



'^('o Williston's romnrks l)olow, p. 980. 



