582 INSECTIVORES xxn. 1 



The order Insectivora is thus a convenience rather than a natural 

 group. 



The full dentition of is usually preserved, and the cusps have 



diverged little from the tritubercular-tuberculo-sectorial pattern but 

 two extra cusps are often added on the outer side of the tooth to make 

 a W pattern ('dilambdodont'). The skull (Fig. 357) shows many 

 primitive features. The orbit is broadly continuous with the temporal 

 fossa (except in tree shrews). There is an incomplete bony palate and 

 an open tympanic cavity, in which the tympanic bone (the old angular) 



forms a partial ring. Non-primitive 



____^-r£5^r^ ;£ ^ characters include the incomplete 



^^^^^^^^■^^r^^)l bony palate of the hedgehog and 



/_/. i %t~\j "Jm^ _yfff the loss of the zygomatic arch in 



ffa^^?^^ tenrec and the shrews. In the post- 



^^^^^^^Ql ?K^f cranial skeleton there is usually 



^^^^^^^^^ found a clavicle, five digits with 



claws in both limbs, and the method 



Fig. 357- Skull of hedgehog (Erinaceus). Q f l ocomo tion IS plantigrade. A 



specialization of the Lipotyphla is 

 reduction or elimination of the pubic symphysis. In the soft parts the 

 primitive characters again predominate. The stomach is simple. The 

 brain has large olfactory bulbs and small cerebral hemispheres, com- 

 posed mainly of large pyriform lobes (rhinopallium), usually not 

 covering the corpora quadrigemina or cerebellum, and with little 

 convolution. The neopallium and corpus callosum are small. Besides 

 the nasal receptors insectivores have a sensitive snout often drawn out 

 into a short trunk. There are vibrissae and acute hearing (especially 

 in moles). The eyes are large in Menotyphla but smaller in Lipotyphla, 

 sometimes rudimentary (moles). In many species the retina contains 

 only rods but there are cones in some species of Sorex and in the tree 

 shrews, Tupaia. Some insectivores retain the cloaca. The uterus is 

 bicornuate and the testes are never fully descended into a scrotum. The 

 placenta is discoidal and haemochorial, that is to say of a type not 

 obviously close to the presumed ancestral mammalian condition. 

 Numerous young are produced (up to 32 in Tenrec). Many insectivores 

 hibernate in winter and are provided with special reserves of fat for 

 this purpose. Most insectivores are solitary but some have social habits, 

 exchanging auditory and olfactory signals {Solenodoii). They may make 

 simple nests. 



*Deltatheridinm is a fossil insectivore found in the Upper Cretaceous 



