MAY, 1921. AMERICAN MARSUPIAL, C^ENOLESTES OSGOOD. 107 



basally, becoming cartilaginous and slender in their distal two- thirds. 

 The thyrohyals are flattened bars standing nearly at right angles to 

 the general axis and ossified to their tips which are united by a short 

 cartilage to the osseous wing of the thyroid cartilage. 



Vacuities. The palate is highly fenestrate, its bony floor being 

 relatively less extensive than in any other living marsupials with the 

 possible exceptions of some of the peramelids. There are two pairs of 

 vacuities of constant occurrence and fairly regular form. The anterior 

 pair extend from the front of the second lateral incisor to the front of 

 the middle premolar and occupy nearly all the space between the tooth- 

 rows. They are divided by the median processes of the maxillaries and 

 premaxillaries which are flattened ventrally to an extent about equalling 

 one-third the width across the paired vacuities. These vacuities are thus 

 about equal to the combined length of the two separate pairs found in 

 peramelids and they are much longer than the single pair of most other 

 polyprotodonts although they are rather closely approached by Smin- 

 ihopsis. The posterior vacuities begin at the front of the last premolar 

 and reach nearly to the transverse ridge bounding the palate. Their 

 endings are almost exactly at the level of the back of the last molars. 

 They are divided by a narrow median extension of the maxillaries and a 

 similar one from the palatine, the two meeting midway of the length of 

 the vacuities and forming a slight expansion at their union. Laterally 

 the boundaries of the vacuities are slightly irregular but anteriorly they 

 are always evenly rounded and the general shape and extent in different 

 specimens is very constant. 



The palatal vacuities are regarded as secondary developments and, 

 since they vary so widely throughout the marsupial group, it is not 

 probable that their character in Canolestes has any special significance. 

 The palate in extinct caenolestids is only imperfectly known. 



A prominent vacuity on each side of the face directly above the 

 infraorbital foramen is one of the most unusual features of the skull of 

 Caznolestes. It is bounded by the nasal, frontal and maxillary bones 

 and opens into the large sinus between the naso-turbinal and the 

 maxillary. Its relations to the overlying dermal tissues are simple and 

 no glandular or other specialized development is apparent. As noted 

 by Thomas (1895), a vacuity in this part of the skull is found in other 

 mammals only among ungulates. 



Auditory ossicles. The malleus has the general features usual in 

 marsupials. The neck is relatively short, the lamina very thin, and the 

 processus muscutaris is but feebly indicated. The manubrium is about 

 two-thirds as long as the processus gracilis and forms a thin flattened 

 blade with a pointed apex and a slightly falcate outline when seen from 



