— 4. _a._ Skull of _Lutra Platensis_. — 4. _b._ Under view of ditto. — 4. _c._ Side view of fore part of ditto. — 4. _d._ Upper view of lower jaw of ditto. — 5. _a._ Skull of _Didelphis elegans_. — 5. _b._ Under view of ditto.[1] — 5. _c._ Side view ditto. — 5. _d._ _Ramus_ of lower jaw, outer side. — 5. _e._ The same, viewed from above, and magnified. INDEX TO THE SPECIES. Page Abrocoma Bennettii 85 —— Cuvieri 86 Auchenia Llama 26 Canis antarcticus 7 —— Azaræ 14 —— fulvipes 12 —— Magellanicus 10 Cavia Cobaia 89 —— Patachonica 89 Cervus campestris 29 Ctenomys Braziliensis 79 Dasypus hybridus 92 —— minutus 93 Delphinus Fitz-Royi 25 Desmodus D’Orbignyi 1 Didelphis Azaræ 93 —— brachyura 97 —— crassicaudata 94 —— elegans 95 Dysopes nasutus 6 Felis domesticus 20 —— Pajeros 18 —— Yagouaroundi 16 Gallictis vittata 21 Hydrochœrus Capybara 91 Kerodon Kingii 88 Lagostomus trichodactylus 88 Lepus Magellanicus 92 Lutra Chilensis 22 —— Platensis 21 Mus arenicola 48 —— bimaculatus 43 —— brachiotis 49 —— Braziliensis 58 Mus canescens 54 —— Darwinii 64 —— decumanus 31 —— —— var. Maurus 33 —— elegans 41 —— flavescens 46 —— fuscipes 66 —— Galapagoensis 65 —— Gouldii 67 —— gracilipes 45 —— griseo-flavus 62 —— Jacobiæ 34 —— longicaudatus 39