Page:Notes on Osteology of Baptanodon. With a Description of a New Species.pdf/8

Pelvic Element.

With specimen 603 in the collection of the Carnegie Museum is a portion of a Fig. 11. (1), ex-ternal view of the acetabular end of the ilium of Baptanodon discus (No. 603), one half natural size, a., articular end ; (2), view of articular end of same; ( 3), cross sec-tion of the broken end. bone which at the time of preparing my memoir on Baptanodon I was unable to identify but since have concluded that it represents the acetabular end of the ilium. This end is expanded into a thickened head somewhat roughened on the acetabular surface. The shaft above the head is constricted up to the fractured end, both antero-posteriorly and internally. One side of the bone is flattened and probably represents the internal surface. The fractured end is nearly half oval in cross-section. (See (3), Fig. 11.) If correctly determined this is the first evidence we have of the character of the pelvic region in Baptanodon and it appears to indicate a weak posterior extremity as compared with the strong anterior limb. It also furnishes additional evidence that Professor Marsh was mistaken in his identification of the limb in the type of B. discus (1955)^[1] as a posterior extremity, a question dis-cussed in my previous paper. This element resembles somewhat the ilium of Toretocnemus californicus, a Triassic Ichthyosaurian described by Dr. J. C. Merriam.

Measurements.

No. 603. Greatest width antero-posteriorly of acetabular end....................................................................................................................................................................................................................................................45 mm.

" 603 " " " " fractured end....................................................................................................................................................................................................................................................27 mm.

Baptanodon robustus sp. nov.

The type material of this species includes a fairly well-preserved pectoral girdle, a series of ten cervical vertebræ beginning with the atlas ; a second series of eleven vertebne from the anterior dorsal region commencing back of the point where the diapophysis becomes distinct from the neurapophysial articular surface ; a third section of eleven posterior dorsals beginning just back of the first vertebræ having the diapophysis and parapophysis united to form a single node-like articulation for the single headed-ribs of this region. The fourth and last section contains parts of twelve anterior caudals. These show the rapid decrease posteriorly in the size of the centra, which has been previously pointed out by Knight.^[2]

This specimen, No. 919, is from the Red Fork of Powder River, Big Horn County,

1. Catalogue number of Museum of Yale University.
2. Knight, W. C., "Some Notes on the Genus Baptanodon with Description of a New Species," Amer. jour. of Science (4), Vol. XV., 1903.