24 



tTNlTED STATES NATIONAL MUSEUM BULLETIN 29 4 



The thin sternum (USNM 23794; pi. 1, fig. 5) of this 

 Choptaiik cetothere is definitely c-ordiform in outline, 

 measuring 99 mm. in width and 91 mm. in lengtii, and 

 the apparent downward bowing when viewed from be- 

 low is attributable to the convex fore and aft curvature 

 of the ventral surface of this bone. The entire bone is 

 rugose, the texture of the exposed surface being granu- 

 lated. Tiie indentation of each side may represent the 

 area of attachment of the sternal end of the first rib. 



The heart-shaped sternum of the Miocene Cetotherium 

 klinden figured by Brandt (1873, pi. 5, figs. 13A, 13B) 

 differs from this Choptank sternum in having an acute 

 posterior end and in lacking the lateral indentation. 

 True (1904, p. 258, fig. 85) has figured a heart-shaped 

 sternum of a North Atlantic Balaena glacialis, but 

 observes that a lack of uniformity among the several 

 recorded specimens is not surprising. 



2. THE VERTEBRAE OF A SECOND MIOCENE CHOPTANK CETOTHERE 



A subsequent tendency toward a rather gradual sliort- 

 ening of the neck seems to lie indicated by a comparison 

 of the relative cervical lengths of Miocene cetotheres 

 with Recent balaenopterid mysticetes. The cervical 

 length of this Miocene Choptank cetothere constituted 

 about 5.6 percent of the total skeletal length. Reduction 

 of the cervical length attributed to the mechanical 

 squeezing of the neck between the head and tlie thorax 

 by water pressure from in front while swimming as 

 advocated by Winge (1918, pp. 62-63; 1921, p. 5) re- 

 quires further consideration. On a purely mechanical 

 basis it would appear more probable that such pressure 

 would liave tended to narrow the skull and possibly to 

 elongate the neck. 



Among these Calvert and Choptank cetotheres fusion 

 of the cervical vertebrae, involving the coalescence of 

 the opposing centra and the pedicles of the neural 

 arches, occurs first between the axis and the third cerv- 

 ical (USNM 11976, 23794). Reduction in the length of 

 the centra is manifested first on the third cervical. In- 

 crease in the length of tlie centra behind the axis is 

 gradual to and including the first dorsal and on some 

 cetothere .skeletons the second dorsal as well. The chief 

 dorsal neck muscles are regarded as an integral part 

 of the musculature of tlie trunk, and function as such 

 during swimming. 



Among Recent balaenopterids, the anterior pairs of 

 ribs have reduced or lost their articular connections with 

 the centra of corresponding dorsal xertebrae; and tlie 

 neck, between the tuberculum and tlie capitulum of tlie 

 rib, has been shortened. Eight anterior pairs of ribs 

 of these cetotheres have definite articular relations with 

 these anterior dorsals. On these dorsals a well-defined 

 articular facet, located at the postcrodorsal angle of 

 the external face of the centrum below the floor of the 

 neural canal, articulates with the capitulum of the fol- 

 lowing rib, and the end of the diapophysis (upper trans- 

 verse process) articulates with the tuberculum. Each 



lateral transverse process (parapophysis) projecting 

 horizontally outward from the centrum on the posterior 

 dorsal vertebrae has a single headed rib attached by 

 ligaments to its extremity. On the posteriormost dorsal 

 the transverse processes rival in size and length the cor- 

 responding processes of the lumbars and are regarded 

 as serially homologous. The neural spines increase in 

 height and width (in an anteroposterior direction) to- 

 ward the posterior end of the dorsal series ; these neural 

 spines are longer and often broader in the lumbar 

 region. 



Atrophy of the hind limb and the accompanying de- 

 generation of the pelvis had been effected in some at 

 least of the cetotheres before the close of the Miocene. 

 Retrogressive remodeling of the innominate bones of 

 the middle Miocene (Astoria fm.) Copliocetus ore- 

 gonensis (Packard and Kellogg, 1934, pp. 58-59, figs. 

 22-24) had proceeded so far that the acetabulum was 

 obliterated, yet the elongated ilium resembled the bal- 

 aenopterid type. There is no visible indication that the 

 pelvic bone retained a functional contact with any ver- 

 tebra. Sacral vertebrae that normally in land mammals 

 possess flattened areas on their transverse processes for 

 a ligamentous attachment of the j^elvis are not recog- 

 nizable as such and they (the sacrals) have assumed 

 the shape and characteristics of vertebrae either in front 

 or behind. 



The caudal vertebrae are distinguished by a ventral 

 articulation with a chevron bone which serves to pro- 

 tect the caudal arterial and venous trunks that follow 

 the haemal groove on the under side of each centrum. 

 Four or five modified terminal caudals are embedded in 

 the caudal flukes of Recent mysticetes and the corre- 

 sponding cetothere caudals are similarly degenerate. The 

 anterior caudals have robust centra, diminishing in size 

 toward the more obviously anterojOTsteriorly compressed 

 centra of the terminal vertebrae and of these the first 

 six or seven have strong neural arches that support neu- 



