CETOTHERES FROM THE MIOCENE CHOPTAJTK FORMATION 



cavation of the internal end of the posterior process 

 above the cliannel for the facial nerve and the develop- 

 ment of a shallow depressed, smooth surface (vertical 

 diameter, 15-21 mm.) on the laosterior face of the pars 

 cochlearis above and behind the foramen rotunda as 

 well as an outward extension of this smooth depression 

 on the surface behind the fossa for the stapedial muscle. 

 On the internal third of the ventral surfac« of the 

 posterior process the facial nerve on its outward course 

 traverses a deep groove, which becomes indistinct be- 

 yond this region as the result of the rather mai-ked flat- 

 tening of this surface. Although somewhat triangular 

 in cross-section, the vertical thickness of the posterior 

 process decreases from the internal end to the external 

 end. The basal portion of the posterior pedicle of the 

 tympanic bulla remains ankylosed to the anterointemal 

 angle of the posterior process of the periotic. 



From a v^entral or tympanic view (pi. 2, fig. 4) the 

 fenestra ovalis is entirely concealed by the overhanging 

 external face of the pars cochlearis and by its almost 

 vertical position ; it is separated from the orifice of the 

 Fallopian aqueduct and the groove for the facial nerve 

 l)y a thin rim. Behind the fenestra ovalis there is a small 

 stapedial fossa which extends downwards also on the 

 external face of the pars cochlearis. An ill-defined shal- 

 low concavity for the reception of the head of the mal- 

 leus is located behind the basal portion of tlie attached 

 slender anterior pedicle of the bulla and in front of 

 the orifice of the Fallopian aqueduct. A small pit serves 

 as the fossa incudis. 



The pars cochlearis is relatively small; the internal 

 half of its ventral surface is irregularly depressed 

 below the narrower convex or inflated external half. On 

 the cerebral face of the pars cochlearis (pi. 2, fig. 5) the 

 centrally located circular internal acoustic meatus is 

 actually smaller than the cerebral aperture of the Fallo- 

 pian aqueduct. The vestibular aqueduct opens into a 

 deep, narrow, elongated fossa behind and above the 

 internal acoustic meatus. Ventral to this vestibular 

 aperture is the small orifice of the cochlear aqueduct. 

 The entire cerebral surface above the internal acoustic 

 meatus, including the anterior process is unusually 

 rugose, pitted and ornamented with short bony spicules. 

 The short anterior process is compressed from side to 

 side, somewhat convex and creased externally, but flat- 

 tened internally. 



See table 3 for measurements of the periotic. 



AUDITORY OSSICLES 



Association of the three little inner ear bones with the 

 tympanic bulla and periotic has rarely been observed 



Table 3. — Measurements (in mm.) of the periotic, USNM $3794 



Right Left 



Greatest dorsoventral depth of periotic, from 

 most inflated portion of tympanic face of pars 

 cochlearis to most projecting point of cerebral 

 face 37 38 



Distance between epitympanic orifice of Fal- 

 lopian aqueduct and extremity of anterior 

 process 48 47. .5 



Length of posterior process, distance from ex- 

 ternal end to outer wall of groove for facial 

 nerve 128 125 



Distance from external end of posterior process 

 to anterior end of anterior process (in a 

 straight line) 168 173 



among specimens recovered from Miocene geological 

 deposits in Maryland and Virginia. The recovery of a 

 detached malleus, an incus, and a stapes in close associa- 

 tion with the left bulla and periotic, as well as a malleus 

 attached to the right bulla of this Choptank cetothere 

 does not, however, provide additional information rel- 

 ative to the functioning of these auditory ossicles. 



Malleus. — The slender stalk-like anterior process 

 (USNM 23794; pl.2, fig. 7) of the malleus is fused with 

 the anterointemal border of the sigmoid process of the 

 bulla. The nearly vertical large hemicircular facet (pl.2, 

 fig. 1) meets the smaller horizontal facet at a right 

 angle in the usual manner of other mysticetes. At the 

 internal end of the protuberant tubercle ( processus mus- 

 cularis) the little acuminate manubrium is bent out- 

 ward and the scar for attachment of the ligamentary 

 process of the tympanic membrance is located on the 

 ventral surface. Two circular nodules similar in posi- 

 tion to those on the malleus of Parietohalaena palmeri 

 (Kellogg, 1968, p. 188) are present on the anterior face 

 of the head of the malleus. The head of the left malleus 

 measures 12 mm. in length and 9 mm. in width. 



Incus.— The incus (USNM 23794; pl.2, fig. 2) ex- 

 hibits a much closer resemblance to the form of Physeter 

 catodon (Doran, 1878, pl.62, fig. 34) than to that of 

 Metofocetus durinmus (Kellogg, 1968, pl.48, fig. 1), 

 which is characterized in part by its more robust crus 

 longum. Two articular facets divided by a ridge, which 

 comprise the surfaces by which the incus is fitted to the 

 malleus, meet almost at a right angle. Tlie largest and 

 longest facet, which is subcrescentic in outline and shal- 

 lowly concave, occupies the base of the body of the incus. 

 The smallest facet, which is rather deeply concave, is 

 situated at the base of the body on the ventral side. The 

 crus longum is relatively slender, rather abniptly in- 

 curved, and on its extremity it bears a slightly expanded 



