ART. 28 SKULL OF ZARHACHIS FLAGELLATOR KELLOGG / 



In most mammals the ethmoid plate ossifies into a median meseth- 

 moid bone bounded below on either side by an ectethmoid. These 

 ectethmoids develop as the cribriform plate. On each side of the per- 

 pendicular mesethmoid in skulls of the Miocene porpoises Diochoti- 

 clius, Ceterhinops, and Squalodon there is a relatively large aperture 

 through which the nasal branch of the ophthalmic division of the 

 fifth cranial nerve passes. In skulls of living porpoises, like Delphinns 

 and Pseudorca, there is a continuous sheet of bone extending upward 

 to the base of the nasals and from which arises the mesial longitudinal 

 perpendicular strip of bone that constitutes the most dorsal portion 

 of the wall between the nasal passages. In each nasal passage in the 

 skull of Lipotes a fissure appears to separate the longitudinal perpen- 

 dicular plate or mesethmoid superiorly from the laterally placed 

 ectethmoid. Inferiorly, the fissure extends obliquely downward 

 across the posterior wall of each nasal passage. These fissures may 

 represent one of the later stages in the closure of foramina similar to 

 those in the ZarTiachis skull. The thin longitudinal bony partition 

 may represent either a dorsal continuation of the combined ecteth- 

 moids or the mesethmoid. Most writers in recent years have held 

 that the pluglike porous bone, which rests in the trough of the vomer 

 and terminates the mesorostral gutter, consists of the presphenoid 

 below and the mesethmoid above, the bones being so intimately fused 

 with each other that their limits can not be defined with any degree 

 of accuracy. Such an interpretation of the mesorostral plug would 

 place the mesethmoid below the cribiform plate in most porpoises 

 and in others a portion would actually lie behind it. The flat plate- 

 like bone which sheathes the anterior surface of the internal borders 

 of the frontals, conceals the frontal fontanelle, and extends upward 

 to meet the anterior margins of the nasals from below, unquestionably 

 represents the combined ectethmoids in the living porpoises or the 

 cribiform plate of other mammals. The telescoping of the rostral and 

 facial portions of the skull was accompanied by a forward thrust of 

 the presphenoid and a backward thrust of the trough of the vomer, 

 and as a result of one or the other, or possibly both, of the above 

 movements, the lower portions of the ectethmoids were separated on 

 the mid line and overspread the presphenoid laterally. Other bones 

 may be included in these lateral plates and, in the case of ZipMus 

 cavirostris Kernan ® refers to them as including the sj^henoidal 

 turbinals. 



A skull of a young Grampus griseus (Cat. No. 15, 773, division of 

 mammals, U.S.N. M.) from Cape Cod, Mass., shows how the 

 Zarhachis type of orifice may have developed. On this young 



« Kernan, John D., The skull of Ziphius cavirostris. Bull. Amer. Mus. Nat. Hist., vol. 38, art. 11, p. 390. 

 August 1, 1918. 



