UNITED STATES NATIONAL MUSEUM BULLETIN 294 



posterior margin. The bifurcated anterior end of the 

 squamosal, which encloses the dorsoventrally com- 

 pressed foramen ovale (length 21 mm.), is preserved on 

 the right side. 



The external angles of the exoccipitals project back- 

 ward barely beyond the level of the posterior articular 

 surfaces of the occipital condyles. A small cavity on the 

 ventral edge of the exocoipital, limited ititernally by a 

 sharp-edged ridge, marks the area for attachment of 

 the stylohyoid. 



The occipital condyles are large and tJie foramen 

 magnum rather narrow. The articular surfaces of each 

 condyle are more strongly convex from end to end than 

 from side to side and they are separated ventrally by a 

 deep narrow groove. 



See table 1 for measurements of the skull. 



Table 1. — Measurements (in mm.) of the skull, USNM £3794 



Exoccipital width 500 ± 



Transverse distance between outside margins of zygo- 

 matic processes 670 ± 

 Transverse distance between outer margins of opposite 



occipital condyles 176 



Greatest or oblique-vertical diameter of right occipital 



condyle 104 



Greatest transverse diameter of right occipital condyle 67 



Greatest transverse diameter of foramen magnum 56 



Posterior face of right occipital condyle to posterior end 



of vomer 105 



Transverse distance between external margins of lateral 



protuberances of basioccipital 165 



Distance between opposite foramina ovale 270± 



TYIVIPANIC BULLA 



Both tympanic bullae (USNM 23794) were crushed 

 against the corresponding periotic and consequently 

 the anterior and posterior pedicles were completely de- 

 molished. The malleus, however, is attached to the outer 

 lip of the right bulla; the three little inner ear bones 

 were found in the matrix between the left bulla and the 

 periotic. The bulla of this Choptank cetothere is larger 

 and also strongly attenuated anteriorly in contrast to 

 the bulla of the contemporary species (USNM 23636) . 



Except for the loss of the anterior and posterior pedi- 

 cles, the type right bulla is essentially complete. Viewed 

 from the dorsal aspect (pi. 2, fig. 7), the involucnmi is 

 creased transversely by thin grooves, conmiencing in 

 front of the posterior pedicle and continuing as far for- 

 ward as the level of or a little in advance of the front 

 edge of the base of the anterior pedicle. The posterior 

 pedicle seems to have projected chiefly from the in- 



volucrum and was separated by a thin cleft from the 

 blunt posterior conical apophysis. The eustachian outlet 

 of the tympanic cavity is rather wide, but behind this 

 outlet scarcely exceeds in width the gap between the 

 overarching thin outer lip and the involucrum. The 

 width of the involucrum is gradually diminished 

 toward the eustachian outlet. 



The distally rounded and transversely twisted sig- 

 moid process is fused along its anterointernal border 

 with the stalk-like anterior process of the malleus and 

 is separated posteriorly by a thin cleft from the pos- 

 terior conical apophysis. 



Viewed from the ventral side (pi. 2, fig. 8) the pos- 

 terior end of the bulla is noticeably wider than the an- 

 terior end ; tliis somewhat rugose ventral surface slopes 

 from the internal to the external border except poste- 

 riorly at the broad anteroposterior furrow which com- 

 mences at the level of the posterior conical apophysis 

 and is continued upward on the posterior face of the 

 bulla. 



Viewed from the external aspect (pi. 2, fig. 6), one 

 observes that the vertical diameter of this bulla pos- 

 teriorly is greater than anteriorly, and that the poste- 

 rior profile is more nearly vertical in contrast to the 

 rounded anterior profile. 



See table 2 for measurements of the right tympanic 

 bulla. 



Table 2. — Measurements (in mm.) of right tympanic bulla, 

 USNM S3794 



Greatest length 74 



Greatest width 47. 5 

 Greatest depth of bulla on external side, ventral face to 



tip of sigmoid process 58 



PERIOTIC 



When the basicranium (USNM 23794) was excavated 

 each periotic was firmly lodged in the broad groove 

 between the exoccipital and the base of the postglenoid 

 portion of the zygomatic process of the squamosal. 

 Through a fortunate circumstance of preservation it 

 was possible to detach the periotics for examination and 

 comparison. 



When viewed from the ventral side (pi. 2, fig. 4), the 

 most noticeable peculiarity of this periotic is the deep 

 indentation of the posterior profile between the pars 

 cochlearis and the internal end of the elongated poste- 

 rior process. This modification is attributed to the en- 

 largement of an extension of the air sac system ( Eraser 

 and Purves, 1960), which has resulted in the deep ex- 



