MORPHOLOGY OF OPISTHOCOMUS CEISTATUS. 65 



In the Common Fowl this part is very large and deep and is fenestrate, so that the 

 coracoid grooves meet in the middle ; on the sixth day of incubation (Lindsay, op. cit. 

 pi. xlv. fig. 3) the top of the sternum projects in front but little beyond the bottom 

 where the keel is beginning to form. On the seventh day my figures (soon to appear 

 in the Trans. Linn. Soc.) show an imperforate rudiment of the rostrum and a definite 

 keel, projecting forwards in front, not far behind the rudiment of the rostrum. In this 

 half-ripe 1st stage of Opisthocomus the fore edge of the feeble keel is exactly at the 

 middle of the sternum, and does not reach the end ; it is behind the middle if the 

 rostrum is taken into account (PI. VII., PL IX. fig. 1, st.k.). 



This bird, both in this stage and in the adult (see Huxley, ojj. cit. p. 306, figs. 8, 9), 

 is absolutely unique ; the Turkey {Meleagris galloimvo) comes nearest it in this respect 

 as far as I have seen. 



Thus in Opisthocomus the space between the Bird and the Lizard is partly bridged 

 over, the endoskeletal sternum receiving considerable support from the exoskeletal 

 interclavicle, as I shall soon show. Nevertheless, the extension backwards of the 

 sternum, with its spreading metasternal outgrowths and its intermuscular keel, makes 

 even this sternum something very difl'erent from that of a Lizard ; it becomes still more 

 unlike in old age (compare PI. IX. fig. 1 with Huxley's figure, and with the sternum 

 of various Lacertilia in my memoir, pis. ix., x., xi., xiii.). I see a manifestly archaic 

 character in the very forward setting on of the 1st sternal rib (compare my figures with 

 Miss Lindsay's, op. cit. pi. xlv. fig. 2) ; there is scarcely any precostal process [pep.). 

 The articulation of the five pairs of sternal ribs [st.r.) is already complete ; the 3rd is 

 on the most projecting part of the costal margin ; these segments are unossified at 

 present and are quite normal. The parts which are developed correlatively with that 

 of the keel for special avian muscular attachment are such as are seen in a large number 

 of existing Carinate birds. These metasternal processes (m.st.) are, with the keel, 

 "additions of later phylogenetic date" than the costal margins (Lindsay, op. cit. p. 710, 

 fig. 4). In this embryo, and in the adult figured by Prof. Huxley, the outer and inter- 

 mediate metasternal (or xiphoid) processes are only partially divided by a small, oval 

 fenestra. But in the mounted Hunterian specimen (A) this part is notched by a gap 

 5 millim. deep and 5 millim. wide ; the inner notch is nearly of the same size, but is 

 more angular. These modifications of the metasternum in individuals is interesting in 

 this type ; so, also, is the want of symmetry in the two moieties of the sternum, as is 

 seen in the median process of this first stage (PI. IX. fig. 1, m.st.). I have no stage 

 showing the mode of ossification of the sternum ; but I strongly suspect that it takes 

 place by distinct ectostoses, as in the Ratitse, the Hemipods, and the Fowls^ 



' In my work on the Shoulrler-girdle and Sternum (pis. xvi., xvii.) I have detected, an error in my description 

 and figures of the sternum of Turnix (p. 184, pi. xvi. figs. 13, 14). The so-called coracosteon was formed by 

 accident; the preparation is still by me, and I see now that what appeared to be a suture is merely a 

 fracture. 



