On the homology of the alisphenoid

The relationships of the elements of the cavum epiptericum in a hypothetical primitive mammalian precursor are reconstructed, and these are analysed in relation to the development of recent mammals, especially the fruit bat Nyctinomus johorensis. The alisphenoid in mammals is part cartilage bone, part membrane bone. The mammalian homologue of the primitive reptilian processus ascendens appears to be internal to the maxillary nerve. If so, then the 'lamina ascendens', that portion of the alisphenoid of mammals which lies between maxillary and mandibular nerves, cannot be a true processus ascendens but must be neomorphic. It is suggested that the mammalian lamina ascendens arose from an upgrowth of the root of the quadrate ramus of the epipterygoid in cynodonts, separating foramen rotundum from foramen ovale. In Ditremata the alisphenoid is completed by an element of membrane bone; this, it is suggested here, originated as the anterior lamina of the periotic in cynodonts, which is retained in monotremes. It is suggested that the alicochlear commissure of mammals originated as the later flange of the periotic in cynodonts.