110 



Mr. W. K. Parker. 



[Dec. 8,. 



forwards by the growing intertrabecula, and are even now in front of 

 the relatively huge " cornna trabecules. " Thus these regions are now 

 well grown in front of the ethmoidal territory, which, instead of being, 

 as in the last stage, in the front margin of the skull, is now fairly in 

 its middle, and this change has taken place whilst the embryo has only 

 become one-half larger — from two-thirds of an inch to an inch in 

 length. It is the hypertrophy of cartilage in the three trabecular tracts 

 that makes the " rostrum " of the Sturgeon so massive, even whilst 

 only a few inches in length, and this state of things exists temporarily 

 in the Garpike. 



Each " cornu " is now like a thick succulent lanceolate leaf, coiled 

 partly upon itself, downwards, at its outer edge ; the middle bar, or 

 intertrabecula, is now half as long as the skull, and projects forwards 

 beyond the cornua, and between the distally-placed olfactory capsules. 

 The cranial cavity is now relatively very large, and is covered by a 

 cartilaginous " tegmen," before and behind ; between these regions, 

 over the huge mid-brain, there is a sub-circular " fontanelle," narrower 

 in front than behind, and emarginate in front by some growth of 

 cartilage in the mid-line. 



The postorbital spike of cartilage has now growu into a narrowish 

 superorbital band, bounding the upper fontanelle and enclosing 

 between itself and the trabecula, below, a large " orbito-sphenoidal " 

 fenestra. The floor is also still open, and this large pituitary space is 

 spearhead-shaped, and is under-floored by the parasphenoid. 



The basipterygoid processes are now well formed ; the basal carti- 

 lages (parachordals) now embrace the huge notochord below, the flaps 

 approaching, but not touching, each other. The notochord is now 

 being formed into a " cephalostyle," but the bony sheath is imperfect- 

 Above, the sphenotic, epiotic, and opisthotic projections of the auditory 

 capsule are more evident, but are not ossified. Some slight bony deposit 

 has appeared in the prootic region. The " cephalostyle " is the first 

 mcfo-cranial bone, and the parasphenoid the first eczfo-cranial centre ; 

 but the exoccipitals are just appearing also. The condyle of the 

 quadrate is now still further behind the middle of the palato- quadrate 

 arcade, the pterygopalatine part of which is now quite free in front, 

 and has grown forwards parallel with the rostrum, as a long style, 

 which reaches further forwards than the cornua trabecules. The 

 " pedicle " of the suspensorium has a facet by which it articulates with 

 the basipterygoid. The mandibular cartilage has also grown equally 

 with the freed pterygopalatine rod, and has now a very large ear-shaped 

 coronoid part in front of and above the articular part. The opercular 

 process and fenestra of the hyomandibular are now formed, but this 

 part is not ossified ; the cerato-hyal diaphysis is present. It is the 

 only shaft-bone in the hyoid arch as yet ; the branchial arches are seg- 

 menting. The superficial bones can now be seen as fine films in the 



