190 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 17, 



which I examined, there was no maxilla whatever, mere fibrous 

 tissue connecting the end of the prsemaxilla with the outer extremity 

 of the antorbital process, and bounding the posterior nasal aperture 

 externally. But in the skeleton of Menobranchus prepared by 

 Hyrtl, now in the Museum of the Royal College of Surgeons, there 

 is, on the right side, a minute bone bearing three teeth, which seems 

 to be a rudimentary maxilla. 



11. The squamosals (Sq) are long slender bones, which extend 

 from near the extremity of the epiotic processes to the articular 

 surface for the mandible on the extremity of the suspensorium. 

 The whole bone is shaped somewhat like a boomerang — the half 

 which lies against the outer side of the suspensorium being bent at 

 an obtuse angle to the half which is connected with the pro-otic, 

 parietal, and epiotic bones, and which runs parallel with the axis of 

 the skull. Where the cranial and suspensorial portions of the bone 

 meet, there is given off, from its posterior margin, a short osseous 

 process, which is directed towards the stapes, covering over the 

 ligamentous fibres which connect the stapes with the suspensorium. 



12. The dentary (D) occupies the whole length of the mandible, 

 rising up on its outer side into a high plate, curving inwards to the 

 symphysis in the middle line, and extending as a shelf, grooved above, 

 beneath Meckel's cartilage, which is received into the groove. Four- 

 teen conical teeth are borne by this bone, and are ankylosed with it. 



13. The splenial (Spl, Plate XXXI. fig. 7) lies on the inner side 

 of Meckel's cartilage, and bears six teeth. 



14. The only other ossified member of the cranial, or facial, series 

 present in Menobranchus is the second basibranchial (Bb", Plate XXX. 

 fig. 2), a styliform bone, broader in front than behind, which lies 

 in the middle line, and articulates, in front, with the two anterior 

 cerato-branchials (Cb l ). 



The chondrocranium, in and upon which the bones now enu- 

 merated and described are developed, is a structure of an exceedingly 

 remarkable character. The whole extent of the chondrocranium was 

 ascertained by decalcifying the skull, macerating it afterwards in 

 glycerine containing caustic potash, and then, partly, viewing it as a 

 transparent object, and, partly, examining sections. 



In the side walls of the depressed cranial cavity, between the para- 

 sphenoid and the second process of the parietal bone (Pa~), there lies 

 a rod of cartilage, which, at the anterior boundary of the orbit, is 

 connected with the tapering antorbital process (A.O). It then 

 bends inwards (on the inner side of the olfactory sac) ; and, meeting 

 its fellow in the middle line, the two proceed, parallel with one 

 another, to the end of the snout, their free extremities being 

 embraced by the pramiaxillse. 



The antorbital process is separated from the chief mass of this 

 rod, which corresponds with one of the trabecules of the ordinary 

 embryonic vertebrate cranium, by a line of fibrous tissue. In the 

 internasal region, on the other hand, the anterior ends of the cartila- 

 ginous rods (which answer to the cornua of the trabecule) become 

 fused together. 



