90 bulletin: museum of comparative zoology. 



for short as the processus conicus. Beauregard terms it " apophyse 

 conique posterieure," Boenninghaus " processus medius." The conform- 

 ation of the various parts just indicated is worthy of particular note 

 since they furnish important diagnostic characters. 



The anterior process is constricted off from the superior margin of the 

 outer lip by a well-marked groove so as to form a slender tenon-like 

 projection (6), which extends backward as far as the middle portion of 

 the periotic. It is, indeed, partially concealed by the latter bone, with 

 which it fuses after first becoming enlarged into a bulbous or knob-like 

 " accessory ossicle," as it is called by Lydekker. This enlarged portion 

 fits snugly into a corresponding cavity of the periotic, and is usually 

 broken away with that bone, instead of with the tympanic, when the 

 two are forcibly separated. Frequently it is lost in the process of 

 fossilization or maceration, when its appropriate cavity is plainly visible 

 in the periotic. It is to the tympanic, however, that the ossicle 

 properly belongs ; and since, according to Boenninghaus, its function is 

 to support the Eustachian canal, it may be compared with the processus 

 tubarius tympanici of the horse and sheep. 



The posterior process (4) is confluent below with both lips of the 

 bulla, is constricted in the middle, and has an enlarged and elongate 

 upper portion or "head." This upper portion exhibits on the cerebral 

 face a broad articular facette for union with a corresponding process of 

 the periotic, and serves externally for the attachment of cartilage or 

 connective tissue, by means of which the combined tympano-periotio 

 is firmly held in place on the under side of the skull. The posterior 

 process has a more open or fibrous structure than other parts of the 

 element to which it belongs. This condition is not only visible exter- 

 nally, but is very conspicuously shown by the articular facette just 

 mentioned. The coarse striation of this surface contrasts strongly with 

 the smooth appearance of the corresponding parts in toothed whales, 

 and is occasioned by a fan-shaped arrangement of bundles of osseous 

 tissue. Spanning across both lips of the bulla near its hinder extremity, 

 the posterior process extends forward so as to come very nearly, but not 

 quite in contact with the processus sigmoideus. The three processes 

 we have named, posterior, conicus and sigmoideus, enclose between 

 them the rounded aperture across which is suspended the membrana 

 tympani (8). The walls which they together take part in forming are 

 interpreted by Boenninghaus as a rudimentary osseous external auditory 

 meatus. 



Superior face (Fig. B). — The more conspicuous body appearing from 



