MOEPHOLOGT OF OPISTHOCOMUS CEISTATUS. 53 



cartilage crop up wherever they are needed in the mobile upper face. The quadrate 

 or upper segment of the mandibular arch (an epibranchial element) is, in this bird, 

 all we have above the gape. In the early stage (Pis. VII., VIII. fig. 1, q.) this is a 

 large segment of cartilage, the body of which has a bony shaft. The true apex or free 

 pedicle (orbital process) is flat and rounded ; the secondary apex or otic process is 

 rounded, and fits in between the squamosal and the anterior margin of the auditory 

 capsule, close in front of the anterior ampulla. As in birds generally, but not as, as a 

 rule, in the Katitse, nor in the Tinamidse, the lower articular facet is bilobate ; the 

 outer lobe is somewhat in front of the inner ; inside the latter there is a knob for the 

 pterygoid, and outside the former there is a cup for end of the quadrato-j ugal. The 

 endoskeletal part of the lower segment, or primary mandible, is in a very instructive 

 condition in these chicks. In the youngest embryo (PI. VII., mk.) Meckel's cartilage 

 is exposed on the inner side up to its fore end ; but in the largest (PL VIII. fig. 2) 

 it is enclosed by the dentary (d.) and has what the other has not yet, namely, an 

 ectosteal plate, the outer ossification of the articulare {ar.) ; this is a thin, deep, lanceo- 

 late tract of ossified perichondrium ; the endosteal part of the articulare has not yet 

 appeared. As bearing upon the order in time of the appearance of these bony 

 deposits, I may remark that in the half-grown Green Turtle (Chelone viridis) the 

 endosteal centre is not present ; in old Turtles it is large. The Anura have only a 

 dentary and a large ectosteal articulare as a rule ; but in the skull of one of the most 

 feebly ossitied types, Bombinator igneus, I find, contrary to rule, the endosteal part of 

 the articulare. The thick malleal part of the primary mandible in Opistliocoinus is 

 generalized ; it is more like that of a Plover than that of a Fowl, for the posterior 

 angular process (p. ffl^.) is a very small hook; the internal angular process (i.ag.) is 

 large, thick, and normal. The development of the posterior angular process is greatest 

 in that gigantic Fowl, the Cock of the Woods {Tetrao urogallus), but it is large in all 

 the true Fowls ; in the Anatidse, which in some things are marvellously like the Galli- 

 nacese, and also in the Flamingo and anserine Ibis, it is large. Thus in this part of 

 its structure Opisthocomus is seen to have fallen short of the Fowl type ; it is a fre- 

 gallinaceous bird, like the Tinamou ^. 



The hyoid arch (Pis. VII., VIII. fig. 3) is quite normal and similar to that of the 

 Common Chick; the distal part of the second arch is merely the lower half of a 

 ceratobranchial rod, with no terminal or hypobranchial segment. The median 

 element (b.hy.) is formed in the usual way by the fusion and partial separation from 

 the ends of the ceratohyals {c.hy.). The rest of the basal tract behind is a single rod 



' The first visceral arch in the Amphibia and Sauropsida generally is chondrified at first as two tracts, 

 an epibranchial and ceratobranchial. In the Salmon (Phil. Trans. 1873, pis. 1-3) all the visceral arches are 

 developed as continuous bands of cartilage, and are segmented afterwards. The same thing takes place in the 

 mandibular arch in Marsupials (o. g. Macropus major) ; thus the incus and maUeus are continuous at first and 

 become separate after birth. 



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