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THE AMERICAN NA TURALIST. [Vol. XXXVI 1. 



2. Zygomatic arches not extended outside the greatest width 

 of the skull.— All projections from the usual regularly shaped 

 skull become reduced. The zygomatic arches are very slender 

 {e.g. Condylura, Talpa, Krinaceus, Orycteropus). 



3. Prenasal ossicle. — A small prenasal ossicle is developed 

 at the tip of the mesethmoid cartilage in Talpa as it is in the 

 pig (Sus), due doubtless to the same cause, that of using the 



4. Incisor teeth chisel-shaped, projecting forward.— In some 



in others they are used as an aid in digging. The wombat 

 (Phascolomys) for example has a pair of strong chisel-shaped 

 projecting lower incisors. Among the rodents Spalax typhlus 

 for example has the well developed lower incisors projecting 

 beyond the lower jaw; the bamboo rat (Rhizomys) uses its 

 incisors to aid in digging, in the Hathyergithr the entrance of 

 earth is prevented by the upper incisors which extend in front 

 of the closed lips. 



5. Cervical and lumbar vertebra more or less fused.— The 

 fusion of these vertebra gives strength and firmness in pushing 

 {e.g. Notoryctes and the armadillos). It is possible that the 

 peculiar intercentral ossicles observed below the lumbar verte- 

 bra especially in Talpa and the Erinaceidae may be of use 

 in strengthening the spinal column. Phascolomys and the 

 Dasypodidce have the cervical vertebrae wide and depressed ; in 

 the latter several of them are commonly anchylosed. In Talpa, 

 the fourth, fifth and sixth cervical s are much lengthened and 

 overlap each other. 



6. Transverse processes of lumbar vertebra very short. — 



from side to side movement in the lumbar region, the corre- 

 sponding muscles and their attachments, the transverse processes 

 are feebly developed (e.g. Krinaceus). 



7. Saerats fused.— The main stress in pushing comes on 

 the sacrals; in the majority of fossorial forms (e. g. Condylura, 

 Lepus) they fuse even to a greater extent than in cursorial 

 animals, as no lateral or vertical displacements but only rigidity 

 are required of them. 



