12 PROCEEDINGS OF THE NATIONAL MUSEUM. voi>. 63. 



The thick convex involuted portion of the tympanic (pi. 6, fig. lo) . 

 or involucrum, is slightly and unequally depressed below the level 

 of the overarching outer lip, and rather abruptly decreases in thick- 

 ness anterior to the furrow on the outer lip, while in Cetotherium and 

 Megaptera it gradually decreases in thickness to the anterior or 

 eustachian angle. The dorsal surface of the involucrum shows the 

 flattened or gently convex undulation which characterizes the tym- 

 panic bone of the baleen whales. 



MEASUREMENTS OF THE TYMPANIC BONE. 



mm. 



Greatest length of bulla 56 



Greatest width of bulla 32 



Greatest depth of bulla on internal side 31.7 



Distance from antero-internal end of tympanic to anterior end of involu- 

 crum 23 



DESCRIPTION OF TWO TYMPANICS. 



Cat. No. 10722, Division of Vertebrate Palaeontology, U. S. National Museum. 



As remarked above, practically all of the tympanic bones which 

 are discovered are imperfect; the thin brittle outer lip which bends 

 over the thick rounded involucrum or inner lip is usually damaged 

 even in the best preserved specimens. However, a pair of tympanies 

 in an exceptionally fine state of preservation were collected by Wil- 

 liam Palmer. Their size and proportions suggested comparison with 

 ParietohalaeTia. The best preserved tympanic bone (pi. 6, figs. 2rt. 

 2?)) measures 52 mm. in length. This tympanic resembles Parieto- 

 halaena (pi. 6, fig. la) in the relative thicknesses of the convex and 

 the concave portions of the involucrum, in the contour of the 

 eustachian end of the cavity, in the proportions of the posterior coni- 

 cal apophysis (pi. 6, fig. 16), and in the general outlines of the tym- 

 panic as a whole. This left tympanic is sufficiently entire to show 

 the form of the tympanic cavity which is bounded by the overarch- 

 ing outer lip, and the size and direction of its anterior outlet or tym- 

 panic aperture of the eustachian canal. 



The anterior process of the right tympanic (pi. 6, fig. 36), which 

 unites with the periotic, is broken off at the level of the outer lip. 

 There is a deep groove on the tympanic between the processus sig- 

 moideus and the so-called posterior conical apophysis of Beauregard. 

 This apophysis is rounded and projects but slightly above the su- 

 perior face of the involucrum. 



The posterior process projects mainly from the involucrum, al- 

 though the outer lip posterior to the conical apophysis contributes 

 the thin edged outer margin. The tympanic cavity is continued 

 forward without interruption to the anterior end of that bone and 

 the outlet is relatively narrower than in Idiocetus laxatus}'^ 



" Van Beneden, P. J., Description des ossements fossiles des environs d'Anvers. Part 

 5. Annales du Mus^e Royal d'histoire Naturelle de Belgique, Bruxelles, vol. 13, pi. 56, 

 figs. 3, 10, 1886. 



