A comparative study is made of external features of the skull in the recent genera of Erinaceidae. An account of the dentition and occlusal relations is also given, including some characteristic features of the root-pattern, hitherto undescribed.
A description is given of the Oligoccne and Miocene Erinaceid material in the British Museum (Natural History). This includes a well-preserved cranium of Neurogymnurus cayluxi which shows the structure of the tympanic cavity, the orbitotemporal region and the ethmoturbinals.
Neurogymnurus minor Lydekker is transferred to the genus Galerix. A new genus (Mioechinus) is created for Erinaceus oeningensis Lydekker, and Erinaceus sansaniensis Lartet is provisionally placed in this genus. The name Amphechinus Aymard is substituted for Palaeoerinaceus Filhol on grounds of priority, and certain differences between specimens from the L. Oligocene, U. Oligocene and L. Miocene are noted.
The inter-relationships of the genera of Erinaceidae are discussed, and the conclusions reached differ in some respects from those of Leche (1902). Echinosorex (Gynmura) is rather closely related to Hylomys, but specialized in various characters, though showing a more primitive arrangement of the anterior teeth. Galerix, which appears to resemble Hylomys in its skull, is probably not far from the ancestry of the recent Echinosoricinae. Lanthanotherium shares with the recent Echinosoricinae a characteristic type of M3. Tupaiodon, which possesses the most primitive upper molar pattern known in the family, is probably related to Galerix. Neurogymnurus is placed in a new subfamily, Neurogymnurinae, on account of the structure of the mastoid region of the skull. The North American genera Brachyerix and Metechinus are not closely related to the Erinaceinae, but should probably be placed in the Neurogymnurinae. The recent Erinaceinae are probably derived from Amphechinus, the enlarged incisors of that genus becoming secondarily reduced. Mioechinus is in a number of respects intermediate between Amphechinus and the recent Erinaceinae. Amphechinus approaches the Echinosoricinae in many cranial characters.
The following tribal names are proposed: Galericini, Echinosoricini, Neurogymnurini, Brachyericini, Protericini, Erinaceini.
The hypothetical Eocene Erinaceidae probably had skulls like Hylomys, and dentitions like Galerix except that the molar pattern resembled Tupaiodon. It is most likely that they were mainly Old World in distribution. The North American genera Entomolestes, Xenacodon and Nyctitherium are considered as possible early Erinaceidae, though probably not ancestral types.
A new family, Metacodontidae, is created for Metacodon, Plesiosorex and Meterix, which are of very doubtful affinities.
The arrangement of turbinal bones in Neurogymnurus is very similar to that of Erinaceus. A scheme of homologies between the ethmoturbinals of Insectivores and Marsupials is put forward, and a new nomenclature suggested.