MORPHOLOGY OF TELROSTBAN HEAD SKELETON. 511 



had been liatched a fortnight, and correspond in other 

 respects to my Stage III, it is evident that there has been 

 some influence at Avork hastening the development of this 

 particular region. The stages during which the otic element 

 should be independent thus seem to have been completely 

 suppressed. On the other hand, such an early connection of 

 the auditory capsule with the basis cranii is a by no means 

 exceptional feature, for in Acanthias (Sewertzoff, 99, p. 284-) 

 it arises as an outgrowth of the latter; and in Sphenodon 

 (Howes and Swinnerton, 01, pi. iii, figs. 1 and o), whilst 

 this region is still pro-cartilaginous, it is quite continuous. 



From the anterior dorsal border of the auditory capsule 

 there arises the pro-cartilaginous post-orbital process (fig. 6, 

 ah. p.) which represents the supra-orbital bar described by 

 Parker in the salmon (73, p. 129), and by Sewertzoff in 

 Carassius (99, p. 312), and is all that is present of the 

 sphenoid region. Of the separate alisphenoid plate described 

 by Sewertzoff in Acanthias (97, p. 413) there is no sign. 



Turning now to the prechordal elements (fig. 1, tr.) of the 

 cranium, the trabeculse are already as well differentiated as in 

 Stohr's salmon. No. III. They are long and rod-like, and 

 chondrified for the greater part of their length. Anteriorly 

 each passes imperceptibly into an expanded plate of pro- 

 cartilnge (e.) which shows signs of being united to its fellow 

 by tissue rich in nuclei. The anterior halves of the trabeculae 

 run parallel to one another; the poflterior diverge (pt.f.) to 

 enclose the infuudibulum and pituitary body, and end in a 

 slight enlargement. 



The relation of the hinder ends of the trabeculte {tr.) to 

 the pai-achordals on the sixth day is somewhat variable. The 

 commonest condition is that in which the trabecula? pass con- 

 tinuously into the anterior end of the parachordals by means 

 of a tract of pro-cartilage. Two seventh- day embryos 

 (Nos. 7, 8) of this stage show a different condition ; in the 

 first both the trabecular are separated from the parachordals 

 by a comparatively wide space (as indicated by the unshaded 

 portion in fig. 1); in the second the trabecula of the one side 



