MARYLAND GEOLOGICAL SURVEY 35 



groove continuous and separate from the dental canal. A genus dis- 

 playing these characters has not been discovered, but I have no doubt 

 that it will be. The new genus Siphonocetus Cope exhibits the groove 

 roofed over by ossification of the gum, and distinct from the dental 

 canal. The genus Ulias indicates that a still farther degeneracy took 

 place, in the fusion of the dental groove and dental canal, while the 

 groove remained open. In Tretulias the same condition persists with 

 the addition that the gingival passages and foramina are present, as in 

 the genus Siphonocetus, and in the later genera. In Cetotherium and 

 in later Balasnidse the groove and canal are fused, the gingival roof 

 is complete, and it is perforated. . It would appear, then, that Ulias 

 .may be descended from the undiscovered genus above mentioned, while 

 Tretulias is descended from Siphonocetus. The exclusively Neocene 

 genera may be tabulated as follows : 



I. Alveolar groove and dental canal distinct. 



Alveolar groove open Not discovered. 



Alveolar groove roofed over and perforate Siphonocetus Cope. 



II. Alveolar groove and dental groove confluent in a gingivodental canal. 



Gingivodental canal open; no gingival canals Ulias Cope. 



Canal open; gingival canals at one side Tretulias Cope. 



Canal with complete and perforate roof Cetotherium Brandt." 



Genus METOPOCETUS Cope. 



" Char. gen. Lateral occipital crests continuous with anterior tem- 

 poral crests which diverge forwards. Frontal bone elongate, not cov- 

 ered posteriorly by the maxillary, coossified with the nasals. Nasals 

 short, coossified with each other, not projecting anterior to frontals. 



'^ Accompanying the cranial fragment on which this genus is founded 

 is a piece of a premaxillary bone of appropriate size, which presents 

 the character of that of a whalebone whale. The true position of this 

 genus is probably between Cetotherium and Agorophius. It is probably 

 a mysticete which approximates the ancestral zeuglodont type which is 

 represented in our present knowledge by the genus Agorophius. It is 

 connected with Cetotherium by the new genus Cephalotropis, which is 

 described below. The three genera form a group, which may be prop- 

 erly referred to the Balsenidee, which is characterized by the elonga- 



