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bulletin: museum of comparative zoology. 



same description will apply to both. We may therefore content our- 

 selves with reproducing one of Denker's illustrations of the latter genus 

 (Fig. G), and referring to his descriptions, as well as those by Beauregard 

 and Boenninghaus, of its auditory organs. The modifications certain 



Fig. G. — Tympanic aspect of right periotic of Phocaena communis Linn. X \ (after 

 Denker). A.t. C F., Apertura tympanica canalis Fallopiae; A. e. A.c, Apertura externa 

 aquaeductus cochleae; C F., Semi-enclosed canal for the nervus facialis, with groove for 

 the muse, stapedius; F. c, Fenestra cochleae; F.p.m., Pit for head of the hammer; P., 

 Promontory, or capsule containing the semicircular canals; P. t. o. p., Processus tympani- 

 cus ossis petrosi; St., Stapes, immovably seated in the fenestra ovalis, but not anchylosed 

 with it by bony union. 



parts have undergone for the better perception of sound waves trans- 

 mitted through the walls of the skull are clearly pointed out by these 

 authors. 



Distinctive Characters of Cetacean Ear-Bones. 

 A brief recapitulation of the more general characteristics of Cetacean 

 ear-bones is here given, by way of directing attention to those features 

 which are available for the distinction of groups of greater or lesser rank. 

 How important these characters really are seems to have been first 

 appreciated by Van Beneden, who remarked in 1885 : " Qui aurait pu 

 se douter que la caisse tympanique et la surface articulaire de la mandi- 

 bule des Mystacocetes auraient pu fournir les caracteres les plus impor- 

 tants pour distinguer des genres et meme les especes 1 " * 



1 Ann. Mus. Roy. d'Hist. Nat. Belg (1885), 9, p. 2. Emphasis is here laid upon 

 the tympanic, because this is the more characteristic bone amongst the Balaenidae, 



