In the Cetacea, which are in other respects osteologically allied to the Pachydermata, the pelvic development suddenly becomes dege nerated into small elongated bones, which may be considered as the homologue of the pubes, and which are imbedded in the muscles of the abdomen immediately in front of the ventral opening, and give attachment to the crura penis. They differ, only in being thicker and less transparent, from the pelvic bones of the true Fishes, between which and the Aram-1mila these animals are the connecting link, as the Bats to the Birds, and the 1110 notremata to the Chelonian reptiles.
In the Dolphins these pubic bones are two simple, elongated,flat bones placed on each side of the median line. In some Whales they are connected by a cross piece, and a.ssurne a hyoid shape (see fig. 257. Art. Cetacea). In the Dugong it is a V-shaped bone formed of four pieces, and articulated to one of the ver tebrx by its free extremities. In the Afanalus, according to Carus, they are entirely wanting.
The pelvic structure of Birds is charac terised by very evident distinctions from the mammalian type, the osseous parts being ac annulated, as it were, on the posterior and lateral parts, leaving the anterior parietes deficient, and being also thinner and more spread out, so as to leave smaller foramina.
The sacrum (fig. 104.. a) is generally broad and large, consisting of from eight to twenty pieces, being increased forwards by ankylosis to the vertebrm corresponding to the lumbar region of the Mammalia, and which contribute to support the iliac wings. This arrangement, as well as the extensive ankylosis of the ilia and ischia, has an evident relation to their bipedal support, and is compensatory for the deficiency of the pelvic circle anteriorly'. It is much more extensive in the Cursores and those which use the legs as the most usual instruments of progression. The bodies of the sacral vertebrx are raised in a continuous ridge on the anterior aspect, those imme diately between the acetabula being larger and broader than the rest (fig. 105. a). The first five or six (s), which may be considered as the ankylosedlumbar vertebrm, present marked spinous processes united in a high crest which intervenes between, coalesces with, and sup ports the iliac wings at their inner margins (fig. 104. b). Their transverse processes, which are also ankylosed to the ilia near their outer borders, are stroncr and well marked on the ventral surface, and differ from those of the true sacral vertebrm in being more pro minently advanced and having a direction more horizontally outwards instead of back wards and upwards (fzg. 105. r), the most pos
terior being the thickest and placed at the junction of the iliac wing with the shaft. A little anterior to the acetabula, however, where the true sacral vertebrx may be con sidered to commence, the spines gradually become less marked as they emerge from between the iliac wings and form a more or less flattened surface (a), which is separated from the coalesced transverse processes by two faintly-marked longitudinal grooves. The transverse processes of the true sacral ver tebrx present a very prominent framework of ridges anteriorly (jig. 105.), which have a direction upwards and backwards as well as outwards, the most strongly marked being opposite the acetabula (b). They are coalesced on the superior aspect, by a thin plate of bone only. The sacrum, as seen from above (fig. 104.), has a diamond-shaped ap pearance, and is marked out from the iliac and ischial elements by a faintly-marked suture (c). The sacrum in Birds is a continuation of the line of the great dorsal curvature.
The coccyx is generally short, composed of from five to nine pieces, generally perforated for the spinal marrow, and curved dorsally, as W have observed before in some Rodents, terminating in a spinous-shaped piece (see fig. 107., a).
The ilia are comparatively short and narrow : with a very short cotylo-sacral rib or shaft (e), directed upwards and forwards, and expanding into a wing (d), concave or grooved supe riorly. The ala is prolonged forwards on the posterior surfaces of the ankylosed lumbar vertebrm, coalescing with their spines and transverse processes; and also extends back wards to a less degree, to coalesce with the upper bifurcation of the anterior extremity of the ischium, in a sort of buttress (f), which projects externally and overhangs the aceta bulum posteriorly, presenting, below, a facet, against which rests the trochanter of the femur, and which is apparently a continuation of the articular cotyloid surface. This but tress is continued from the isehium inwards, as a strong ridge, to the extremity of the strongest of the sacral transverse processes before mentioned (fig. 105. b), opposite the lateral angles of the diamond-shaped sacral plate, and evidently contributes in the greatest degree to support the trunk upon the femurs in the standing posture. The principal part of the ilium in birds is composed of the which lie almost altogether on the dorsal aspect of the spinal column. The total axis of the ilium, however, crosses that of the spine at an angle of from 150° to 160°, and does not, strictly speaking, lie parallel to it, as is com monly asserted (see fig. 112. 10.).