ART. 27 ASYMMETRY IN SKULLS OF MAMMALS HOWELL 3 



There is described and figured by Paravicius^ the skull of a dog 

 exhibiting a marked dextral twist to the rostral region. Unfortu- 

 nately, this specimen lacked the lower jaw, for the characters strongly 

 indicate that the reason for the asymmetrical condition was the sub- 

 normal development of the right ramus of the mandible, due to a 

 fracture during puppyhood. There is no appreciable asymmetry of 

 the temporal fossae. 



Toldt ^ discusses the asymmetrical skull of a fox in the Vienna 

 Museun. On account of a diseased condition of the left, lower, fourth, 

 premolar the animal had used the temporal muscle of the right side as 

 exclusively as possible. The myological condition resulting was a 

 slight increase in the size of the right temporal, shifting its ridge a 

 bit entad. Atrophy, or rather nondevelopment, of the left temporal 

 was marked, its ridge developing considerably laterad of the normal 

 position, and there is practically no lambdoidal crest upon that side. 

 The left supraorbital process and the portion of the temporal fossa 

 immediately caudad thereto, however, are shown in the illustration to 

 be better developed upon the left side. A somewhat puzzling circum- 

 stance is the fact that in the drawing, the left zygomatic arch is shown 

 to be about 2 mm. farther from the cranium than the right, while the 

 text also mentions that the disparity is slight. This state of the left 

 zygoma is at variance with what one would naturally expect to 

 accompany a reduced temporal muscle. I imagine, however, that 

 there was some special disparity between the masseter muscles to 

 account for it — an hypothesis which can hardly now be proven. 



An instance is mentioned by Leisewitz'' of slight asymmetry in the 

 skull of a monkey of the genius Lagothrix. The bilateral disparity 

 is very poorly defined, however, and the author is mainly concerned 

 with consequent slight differences in tooth wear. 



The skulls at hand exhibiting definite asymmetry number four. 

 Two of them are of primates, whose musculature of mastication is 

 adapted to the absence of true glenoid fossae, and hence, to a con- 

 siderable movement of the mandible in all directions. The remaining 

 two skulls belong to a single species of pinniped — a carnivore having 

 true glenoid fossae, which permits practically no lateral nor propali- 

 nal motion of the mandible. 



ASYMMETRICAL SKULLS OF PRIMATES 



The writer is indebted to Mr. G. S. Miller, jr. for permission to 

 study the skull of a form of gorilla in the National Museum collec- 

 tion (see pis. 3, 4, 5, and 6). This bears the data " No. 239883, male, 



sparavicius, Q., Asimmetrie cranio fascial! in un cane, Atti Soc. Ital. scienze nat., 1902, pp, 349-352. 



* Toldt, Von K., Asymmetrische Ausbildung der Schiafenmuskeln bei einen Fuchs infolge einseitigei 

 Kautatiglieit, Zool. Anz , vol. 39, 1905, pp. 176-191. 



^Leisewitz, W., Ein Beitrag zur Kenntnis der bilateralen Asymmetric des SSugetierschadels, Sitzungs 

 berichten der Qesells. fiir Morph. iind Phys. in Munchen, 1906, pp. 1-15. 



