CETOTHERES FROM THE MIOCENE CHOPTANK FORMATION 



25 



ral spines and metapophyses. Neural spines progres- 

 sively reduced in height and width (in an anteroposte- 

 rior direction), large metapophyses spreading obliquely 

 upward and outward, but not interlocking with the 

 vertebra in front, and diminishing neural canals also 

 characterize the anterior caudals. Shortening of the 

 transverse processes terminates in the flange-like condi- 

 tion of the sixth caudal. On the small anteroposteriorly 

 comjDressed terminal caudals, the neural canal is not 

 covered by the roof of the neural arch and chevrons are 

 not attached ventrally. 



Comparison with skeletons of Recent balaenopterids 

 shows that the caudal vertebrae of these cetotheres 

 possess the requisite structural features to support the 

 musculature and tendons required to manipulate effec- 

 tively a terminal organ of propulsion. The cetothere tail 

 obviously was strongly muscled and employed as a pro- 

 pelling organ by up-and-down or side-to-side strokes, 

 or possibly with a sculling motion. The strongly muscled 

 cetothere tail with a terminal flattened horizontal fluke 

 would function as the primary specialized organ to 

 propel the whale forward, upward or downward in a 

 nearly straight line. The pectoral flippers serve in steer- 

 ing and balancing. 



The cordiform shape of the small sternum (pi. 1, fig. 

 5) of a Choptank cetothere indicates that it had become 

 reduced or atrophied and consequently has losrt most 

 of its functional relationship with the ribs in the thorax. 



Halicetus," new genus 



Ttpe-species. — Halicetus ignotus, new species. 



Diagnosis. — Atlas not unusually thickened and a 

 vestigial hyapophysial process present. Odontoid proc- 

 ess of axis short, acutely pointed. Pedicles of neural arch 

 of third to seventh cervicals short and rather wide; 

 neural canal relatively high, not unusually widened; 

 roof of neural canal arched. Neural spines of doi-sals 

 progressively increasing in anteroposterior width and 

 height toward posterior end of series, almost vertical 

 ■ on anterior dorsals in contrast to slight backward in- 

 clination of posterior dorsals. Neural spine of eleventh 

 lumbar shorter but broader than on first lumbar; meta- 

 pophyses of lumbars and posterior doi-sals thin, deep 

 vertically. Transverse processes of first to fourth caudals 

 anteroposteriorly widened toward extremity. Posterior 

 process of periotic greatly enlarged anteroposteriorly; 

 bulbous anterior jjrocess rugose and porous internally, 

 elsewhere irregularly creased or wrinkled longitudi- 

 nally; dorsal rim of circular internal acoustic meatus 

 projects internally (cerebrally) beyond slit-like depres- 



sion for aperture of vestibular aqueduct and the cochlear 

 aqueduct orifice; transverse and vertical diameters of 

 excavation behind stapedial fossa for extension of air 

 sac system approximately equivalent. 



Halicetus ignotus, new species 



Type-specimen.— USNM 23636. Skull when exca- 

 vated badly fractured and individual bones detached; 

 both tympanic bullae; both periotics; seven cervical 

 vertebrae; 12 dorsal vertebrae; 3 lumbar vertebrae; 12 

 caudal vertebrae; rib fragments. Collector, Richard 

 Warren, April 1964 . 



Horizon and locality. — In shell layer of sandy zone 

 19, about 14 feet above beach level, 1.9 miles (10,000 

 feet) south of Calvert Beach Run (U.S.G.S. Cove Point 

 sheet, 1943), Calvert County, Maryland. Choptank for- 

 mation, middle Miocene. 



SKULL 

 TYMPANIC BULLA 



Both tympanic bullae (USNM 23636) were associated 

 with the i^eriotics attached to the squamosal portions of 

 the type skull. The left bulla is the best preserved 

 although both lack the anterior and posterior pedicles. 

 Except for a more accentuated roughening of the dorsal 

 surface of the involucrum by development of wider 

 transverse ridges as well as the greater width of the 

 anterior eustachian outlet of the tympanic cavity, the 

 bulla of this mysticete resembles in several respects the 

 corresponding auditory bone of Pelocetus calvertensis 

 (Kellogg, 1965, fig. 4, p. 12), not only in size. Viewed 

 from the external side, the ventral profile of this bulla 

 (pi. 16, fig. 5) is slightly arched, the posterior end being 

 markedly convex and the anterior end obliquely trun- 

 cated in a dorsoventral direction. The large elongated 

 sigmoid process is twisted at a right angle to the long 

 axis of the bulla, its bluntly rounded extremity being 

 bent backward. 



See table 17 for measurements of the left tympanic 

 bulla. 



The basal portion of the slender processus gracilis 

 (anterior process of Ridewood, 1922, p. 241, fig. 10) of 

 the malleus remains fused to this bulla in the groove 



Table 17. — Measurements (in mm.) of left tympanic bulla, 

 USNM SS6S6 



Greatest length ^^ 



Greatest width ^0 



Greatest depth of bulla on external side, ventral face to 



tip of sigmoid process 



52.5 



' Halos, sea, in allusion to life in the .sea. 



821-614 O— 69- 



