MOEPHOLOGT OF OPISTHOCOMUS CEISTATUS. 75 



a little further backwards than the post-ilium ; behind it is notched above and 

 lobate below. The pubis (pb.) is half the breadth of the ischium, but it is one third 

 longer ; it thickens behind and then narrows to a rounded point. At the last fourth 

 of its length it is still, as in front, unossified ; and this part is twisted upon itself in 

 the same manner as 1 find it in the Guillemot {TIria troile). In that bird the prenasal 

 rostrum and Meckel's cartilages are similarly twisted, as also is the epicerato-hyal 

 band in this bird. If I had found these twisted cartilages in the embryo of a tame or 

 domestic bird 1 should, of course, have put them to the account of Teratology. I 

 cannot do this either in the Guillemot or the Hoatzin ; in these birds, which are in a 

 state of nature, they have manifestly an ontogenetic meaning. In the fossil Toothed 

 Birds (Marsh, ' Odontornithes,' pis. i., xx., xxi., xxii.) the bill is relatively much longer 

 than in average modern birds related to them: Hesperornis is a Colymbine Grebe; 

 Ichthyornis is an ancient Gull ; the length of their jaws bears out this view. In the 

 hyoid arch I have shown that the bird agrees with Hatteria (Huxley, P. Z. S. 1869, 

 p. 397, fig. 4) ; it merely loses part of that arch by absorption, but not without an 

 attempt on its part to grow larger. The rest of the organization forbids this, and it 

 becomes a twisted tape. All these things have but one explanation. 



The forms that among extinct Eeptiles come nearest to Birds in their hip-girdle — 

 namely, the Ornithoscelida — have an exceedingly long pubis and ischium (DoUo, 

 ' Bulletin du Musee Royal d'Histoire Naturelle de Belgique,' tome i. 1882, planche ix. 

 & tome ii. planche v.). The ancestral birds must have come much nearer these 

 Reptiles than modern types ; but these latter all pass in their hip-girdle through an 

 Ornithoscelidan stage after they have lost the general Reptilian condition of this part. 

 The gently modified Ornithoscelidan pelvis of the African Ostrich has its very accurate 

 counterpart in the embryo of the S\^'an {Cygnus), in which bird, for teleological reasons, 

 the postacetabular region of the pelvis is extremely long and the pubes very large, as 

 in the Ostrich, but they are not, as in that bird, fused together below. The hip-girdle of 

 the adult 0])isthocomus has to be studied with the sacrum as part of the pelvis. 

 Prof. Huxley {oj}. cit. p. 308) says: — "The pelvis (figs. 10, 11) is more like that of 

 Coturnix than that of Corythaix ; but though it resembles both, it differs from both in 

 the absence of any ilio-pectinal process, and in the circumstance that the ilio-sacral 

 fossae are completely roofed over by bone. The obturator foramen, as in many Galli- 

 naceous birds, is not bounded by bone behind ; in Corythaix it is: " [bounded, but not 

 perfectly enclosed, by ankylosis of the ischium and pubis, any more than in Gallinaceous 

 birds and Opisthocomus. Moreover, Corythaix has the roofing of the post-ilium over the 

 ilio-sacral fossae, although not to the same extent as in Oinsthocomus]. The pelvis of 

 Opisthocumus is very generalized, and differs very much from that of the Gallinaceae 

 proper (Peristeropodes and Alectoropodes) ; they have the postacetabular region much 

 broader, and the breadth is seen in its most remarkable degree in Cwpidonia cujndo 

 (Shufeldt, OJ}. cit. " Tetraonidse," pi. xii.). In that bird, and in Talegalla lathami, the 



VOL. XIII. — PAET II. No. 5. — April, 1891. m 



