54 PROF. W. K. PARKER ON THE 



(b.h.hr.), and really belongs to the third postoral arch, and to arches that have been 

 suppressed behind that. That third arch is developed as the " cornu major " ; it is 

 merely divided into a ceratobranchial and epibranchial {c.br., e.hr.), the lower pieces 

 ossifying. The upper part of the second arch repeats the old Amphibian specializa- 

 tion ; it is now the skeleton of the middle ear — the stapes or columella. Morpho- 

 logically this part (PL VIII. ligs. 5, 6) is a pharyngobranchial, an epibranchial, and 

 the beginning of the ceratobranchial region ; this is followed by a membranous tract 

 of considerable and, during growth, of relatively increasing extent. Below, the cerato- 

 hyal = ceratobranchial just described, breaks out again. In these arrested and 

 specialized pharyngohyal and epihyal cartilages we have the archaic character seen 

 in Eatteria, and in the Crocodilia for a time, namely, the fusion of these two cartilages, 

 the intermediate and secondary element, the " interhyal," so well known in Ganoids 

 and Teleosteans, binding together the two tracts. The interhyal (=infrastapedial) 

 is present in the ordinary Lacertilia, but it is free below ; it does not catch the top of 

 their long epihyal ; it does not exist in the Amphibia. 



All birds are therefore Hatterian in this respect ; but the embryos of Opisthocomus 

 show this better than any birds I have yet worked out. This specialized hyomandibular 

 is at first of the full relative size ; its growth, however, soon becomes arrested : it has 

 the usual dilated opercular plate ; a short thick shaft, the mediostapedial {m.st.) ; a 

 tongue-shaped extrastapedial (e.st.) ; and a forked suprastapedial (s.st), finished above 

 by a ligamentous tract. The infrastapedial (i.st.), (=interhyal of Fishes and Mam- 

 mals) is developed directly from the columella and ends in a bulbous form ; it is half 

 the length of the extrastapedial, and one-third of its width. Articulated to this is a 

 tract of cartilage as long as the whole columella proper ; this is sharp above, and then, 

 after articulating with the infrastapedial, which it exceeds in size, it turns suddenly 

 backwards, and then makes a second sudden turn forwards, and ends in a large tongue- 

 shaped lobe, which lies on the edge of the basitemporal bone. This cartilage is the 

 epihyal, with a continuous but enlarged rudiment of the top of the ceratohyal L 



' Let this structure be compared with what is seen in Eatteria, and also for a time in the Crocodile (see 

 T. Z. S. vol. xi. pis. 68 & 69). The ectocranium of Opisthocomus conforms very closely to that of Carinate 

 birds generally, but it comes nearest to that of the Gallinaceous tribes. It is a " holorhinal " skull with a 

 strongly ctu'ved rostrum and highly ossified endocranium. The cranial and rostral parts are of nearly equal 

 length. The hinge of the rostrum on the cranium is, however, much more perfect than in the Common Fowl ; 

 and this agrees with the normcd Cracidse, which are very Cuculiue in this respect. The splints of the rostrum 

 or upper face are weU formed already (Pis. VII. & VIII.). but in the first stage the premaxillaries (pa;.) do 

 not cover the prenasal rod {pn.) in front. The nasals (w.) have a round notch behind the alinasal waU ; 

 this skuU is therefore holorhinal. The frontals and parietals (/., p.) are quite normal; the large squamosal 

 («2.) lacks the special jugal spur so well seen in Gallinaceous birds generally. In this respect this bird is 

 generalized ; so it is also as to the lachrymal (?.), which is very small and attached to the back of the nasal. 

 In the palate (PI. VIII. fig. 1) the parostoses are slender and feeble, much Kke those of the GalHnacea; 

 generally. This is especially seen in the feeble state and hidden position of the maxillaries {mx.), which, as in 



