Cope.] 1^^ [Aprils, 



sursiplana, although it is not so sharply defined by a strong transverse 

 convexity of the superior surface, as in the latter. Nor is there as strong 

 a bevel of the anterior extremity of the superior face when viewed from 

 within, as in B. definita. An equally conspicuous difference is to be seen 

 in the form of the inferior wall. According to Lydekker this surface, 

 when the bulla is viewed from within, consists of three planes separated 

 by rounded angles, of which the median is longer than those at the ends. 

 In the B. sursiplana this surface is regularly convex from end to end. In 

 size this species is like that of the large Baloenopterge, including the B. 

 definita. 



Measurements. mm. 



Axial length of bulla 98 



Width at posterior extremity of anterior hook at superior 



border 71 



"Width at anterior extremity of orifice 35 



Width at posterior extremity of orifice 53 



Depth at middle (about) 55 



Greatest depth of lip 38 



Bal^ena affikis Owen, Brit. Foss. Mamm. and Birds, 1846 ; plate opp. 

 p. xlvi. Lydekker, Catal. Foss. Mamm. Brit. Mus., v, p. 17, Fig. 7. 

 Two otic bulla} agree with the figures and descriptions of this species, 

 except in their smaller dimensions. The smallest given by Lydekker is 

 120 mm. Tlie larger specimen in the Johns Hopkins collection is 105 

 mm., and the smaller is 95 mm. 



All of the material described in this paper, excepting the type specimen 

 of the Cetotherium crassangulum, belongs to the Museum of the Johns Hop- 

 kins University of Baltimore ; and I wish to express my obligations to 

 the authorities of that institution, and especially to Prof. William B. 

 Clark, in charge of the Department of Geology, for the opportunity of 

 studying it. ^ 



Explanation of Plate. 



Diagrammatic sections of the left mandibular rami of species of Balseni- 

 dae, one-half natural size. 



Fig. 1. Ulias moratus Cope. 

 Fig. 2. Tretulias buceatus Cope. 

 Fig. 3. Siphonocetus prisms Leid3\ 

 Fig. 4. Siphonocetus clarkianus Cope. 

 Fig. 5. Siphonocetus expansus Cope. 

 Fig. 6. Cetotherium pusiUum Cope. 

 Fig. 7. Cetotherium polyporum Cope. 



Lettering: E. O., External gingival canal; /. O., Internal gingival 

 canal ; O. 0., Gingival groove ; D. C, Dental canal. 



