INTRODUCTION GENERAL CONSIDERATIONS The idea of “Insectivora” serving as a waste-basket taxon for a variety of extinct and extant small, insectivorous mammals has had a relatively long history (Symonds, 2005). For the purposes of this book, we have chosen to recognize three groups of North American insectivorous mammals: the potentially monophyletic Leptictida and Lipotyphla, and the clearly polyphyletic “Proteutheria.” Leptictids and proteutherians are only represented by extinct taxa, while lipotyphlans are abundant and diverse from the Paleocene to the Recent. It is nearly certain that these three groups do not share common ancestry except perhaps at a very basal level within the mammalian tree where insectivore-grade stem lineages may have given rise to a number of different mammalian groups (Asher, 2005). Modern lipotyphlans are represented by nearly 400 species included in four families, Erinaceidae, Solenodontidae, Soricidae, and Talpidae. We consider the families Tenrecidae, Potamogalidae, and Chrysochloridae to be members of Afrotheria and distinct from Lipotyphla (Asher, Novacek, and Geisler, 2003; Springer et al., 2005). Among living mammals, lipotyphlan diversity is eclipsed only by bats (approximately 1000 species) and rodents (over 2000 species). Lipotyphlans are common on all of the northern continents, have modest representation in Africa and Madagascar, and have recently colonized South America (represented by the soricid Cryptotis). Lipotyphlans are lacking in Australia and Antarctica but are present on the islands of the Philippines and the Greater Antilles (Symonds, 2005).