. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Fig. 6. Left, posterior presacral ribs of the right side, X 'A- Rig^^i, left clavicle, external and internal views, X Vi- areas are essentially fused into a single articular surface. A decrease in length of ribs is not marked until approximately rib 23, which is notably shorter than that pre- ceding it (with a length in CNHM 272, as preserved, of 413 mm) and ribs 24-26 are increasingly short, the last being about 158 mm long in CNHM 272. As in pely- cosaurs generally, the freedom of rib artic- ulation with the vertebr
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Fig. 6. Left, posterior presacral ribs of the right side, X 'A- Rig^^i, left clavicle, external and internal views, X Vi- areas are essentially fused into a single articular surface. A decrease in length of ribs is not marked until approximately rib 23, which is notably shorter than that pre- ceding it (with a length in CNHM 272, as preserved, of 413 mm) and ribs 24-26 are increasingly short, the last being about 158 mm long in CNHM 272. As in pely- cosaurs generally, the freedom of rib artic- ulation with the vertebrae decreases pos- teriorly; the last 3 short ribs appear to be well fused and immovable and hence are to be considered as lumbars, and the next pre- ceding may also have been immovable in life, although in 4-0-4 a suture between rib and vertebra was seen during preparation for the fourth presacral. All the more posterior ribs tend to have shafts which are relatively broad and thin for most of their length; the last 5, however, definitely taper to a point distally, and thus lack a sternal connection, and the same may be true of rib 21. As far as segment 23 the ribs continue to be curved, turning downward and backward in life. Ribs 24- 26, however, are nearly straight and di- rected laterally, their length being little more than that of the proximal nearly straight segment of rib 23. The breadth of the proximal part of the ribs increases pos- teriorly to a maximum in rib 24, which is very broad in proportion to its length. Sacral ribs (Fig. 3). In correlation with the width of the trunk and pelvic region, the sacral ribs are longer than in sphena- codonts and ophiacodonts. Three sacral ribs are present. These are tightly fused to their vertebrae over a broad area; this includes the short transverse processes which arise from much of the lateral surface of the centra. No clear sutures are seen, but dor- sally a rugose anteroposterior ridge marks the line of fusion of rib and trans
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Keywords: ., bookauthorharvarduniversity, bookcentury1900, booksubjectzoology
