MIOCENE PORPOISES — KELLOGG 145 



formis Ralfs, Melosira sulcata Ehrenberg, and Sce/ptroneis caduceus 

 Ehrenberg. Calvert formation, upper Miocene. 



MANDIBLES 



The mandibles (pi. 17, fig. 1) are represented by a fairly complete 

 symphysis and a 69 mm. fragment of the left ramus posterior to the 

 hinder end of the symphysis. The upward curvature of the anterior 

 end as well as the greater length of the symphysis distinguish these 

 mandibles from those of Delphinodon dividum. Since the braincase 

 and the hinder portions of both mandibular rami were not preserved, 

 no estimate can be made of the total length of either mandible. Other 

 Calvert Miocene porpoises, with the exception of Zarhachis, Schizo- 

 delphis, Eurhinodelphis^ and related forms, have 11 to 14 teeth located 

 in each mandibular ramus behind the fork of the symphysis. On this 

 basis each mandible may have held from 25 to 28 teeth. 



Although the symphyseal portion of the mandibles is firmly anky- 

 losed, a narrow groove extending the length of the symphysis marks 

 the contact of the opposite rami dorsally and ventrally. The anterior 

 half of the symphysis is bowed upward and its anterior end is obliquely 

 truncated. There are 16 alveolae in the left mandibular ramus, of 

 which 14 are located on the symphysis. The alveolae diminish in 

 size toward the anterior extremity, although those in the right mandi- 

 ble average larger. Two teeth were directed forward and upward at 

 the extremity of each ramus. The largest alveola (the fifth counting 

 forward from hinder end of symphysis) in the right mandible measures 

 9.5 mm. anteroposteriorly and 8 mm. transversely. At the alveolar 

 border the alveolae on the symphysis are separated by thin septal 

 walls and the alveolae in the two series are not located opposite one 

 another. 



The dorsal surface of the symphysis is relatively flat transversely 

 to and including the anterior end, the interval between opposite 

 alveolae being 20.5 mm. at the level of the third pair of teeth (counting 

 forward from posterior end of symphysis) and 13 mm. at level of 11th 

 pair of teeth. 



The largest nutrient foramen is located on the outer surface of the 

 left mandible (pi. 17, fig. 2) 14 mm. below the rim of the fourth 

 alveola (counting forward from posterior end of symphysis) . Smaller 

 foramina are discernible below the sixth and seventh alveolae. A 

 larger foramen is located on the right and left mandibles below the 

 11th alveolae within 3 mm. of the midline of the ventral face of the 

 symphysis. Short grooves extend forward from each of these fora- 

 mina. A cross section of the symphysis resembles a wide, open V. 



