376 University of California Puhlications in Zoologn [Vol.21 



tion of this ethmo-turbinal complex that seems to affect the vomer 

 most profoundly is the mesethraoid, which is composed of two sym- 

 metrical halves that lie side by side and are separated by a mem- 

 brane. The mesethmoid is not present in the skull of B age. In the 

 skull of F age it is present as two unossified septa, and in a skull 

 slightly older than L it is well ossified. Vertically the mesethmoid 

 extends from the vomer to the frontals, and dorsally, it extends 

 longitudinally from the cribriform plate to the naso-frontal suture. 

 It is rounded on the anteroventral border, hence, ventrally, does not 

 project so far anteriorly as it does dorsally. The vomer becomes 

 intimately associated with the ventral border of the mesethmoid and, 

 in adults, practically loses its identity. The cribriform plate is also 

 supported in part by the vomer and perhaps influences its develop- 

 ment. In one specimen, slightly older than L, the cribriform plate 

 contains 187 foramina. These are arranged in three main tracts; one 

 central and one on each lateral portion of the plate. Each lateral 

 tract is divided into four primary regions on each side, but here the 

 symmetry of the two sides ceases. 



Presphenoid. — The presphenoid is cartilaginous in A. In B it is 

 represented by a small ossification which is slightly pointed anteriorly 

 and is in contact only with the palatines. The small size and position 

 of this ossification indicates that the presphenoid develops from a 

 single center, a manner of development that Parker (1885, p. 114) 

 has noted as perhaps a characteristic feature in rodents or at least 

 one that occurs in some forms. He says : ' ' The presphenoid is very 

 rarely an independent ossification ; it attains to that special condition 

 most perfectly in the Marsupials and Rodents; ..." He also says 

 (1885, p. 204) : ". . . It is also present in some of the lesser rodents 

 (at any rate in Arvicola, Mus, etc.)." In a skull from the same litter 

 as F the presphenoid is 7.3 mm. long, and bears the same relations 

 to the surrounding bones as it does in older skulls. However, the 

 presphenoid in this particular skull, throughout most of its length, 

 is transverse in cross-section. One side of this triangle lies hori- 

 zontally and forms part of the floor of the brain case. At the anterior 

 end the sides describe a nearly equilateral triangle, but this shape 

 gradually changes toward the posterior end of the presphenoid. Plere 

 the ventral angle becomes more obtuse and the extreme posterior 

 portion of the bone is flat. This shape is decidedly different from that 

 in a skull slightly older than 31, in which the presphenoid is quad- 

 rangular in cross-section throughout its length. As in the specimen 





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