﻿4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 59 



reaching superior surface ; ending opposite distal end of nasals, or as 

 a narrow slip of bone on each side of them. No temporal ridge. 

 Parietals scarcely appearing on vertex. Frontals very short on 

 vertex. Nasals moderate. Tympanic bone with a ridge on inner 

 side (that is, on the border nearest the median line of the skull). 

 Alveolar groove and dental canal of mandible as in Plesiocetus. 

 Proximal end of mandible solid. Orifice of dental canal small. 

 Alveolar groove roofed. 



AULOCETUS Van Beneden. 1861 



Orbital plate of frontal less descending than in Balcenoptera, not 

 so much produced anteriorly, and with a trace of a temporal ridge. 

 Form of maxillae proximally, and form of parietals and frontals on 

 vertex, as in Balcenoptera. Nasals very long. Proximal end of 

 premaxillae as in Balainoptera, but broader and more superiorly 

 placed. Tympanic bone with a ridge on internal border (that is, on 

 the border nearest the median line of the skull). Maxillae very 

 broad. Premaxillae expanded distally and inserted between maxillae 

 and nasals proximally. Mandible not very convex externally at 

 posterior end, nor emarginate toward inferior border ; not deeply 

 concave internally (hence, as in Balcenoptera). Alveolar groove 

 and dental canal as in Plesiocetus (?). Alveolar groove roofed. 



PLESIOCETUS Van Beneden. 1859 



Orbital plate of frontal not abruptly on a lower level than vertex, 

 trumpet-shaped, not produced anteriorly; with a distinct or indis- 

 tinct temporal ridge. Parietals on vertex long or moderate ; frontals 

 the same. Nasals long. Maxillae rather narrow, without a prox- 

 imal nasal branch, or extension. Proximal end of premaxillae broad, 

 superiorly placed, and articulating with frontals by a very coarse and 

 deep interdigitating suture. Tympanies flat or concave on internal 

 border (that is, on the border nearest the median line of the skull). 

 Proximal end of mandible convex externally, with a more or less 

 pronounced emargination near inferior border ; internally, very con- 

 cave. Alveolar groove and dental canal confluent, the former roofed 

 over. Gingival foramina on the inner side ( ?) . 



Note. — The vertex of Plesiocetus does not seem to be well-known, but as 

 Van Beneden's figures of other parts correspond to those of Heterocetus, etc., 

 it may be safe to assume that the vertex has the same form as in those nom- 

 inal genera. The genus Plesiocetus may be regarded as including Heterocetus 

 Van Ben., Amphicetus Van Ben., Mesocetus Van Ben., Idiocetus Capellini, 

 Isocetus Van Ben., and probably also Herpetocetus Van Ben. and Burtinopsis 

 Van Ben. 



