36 PROCEEDINGS OF THE NATIONAL MUSEUM. vui,. c,:!. 



Although the presternum (pi. 11, fig. 1) of this fossil porpoise is 

 crushed and incomplete, it is evident that it must have resembled 

 Platanista when perfect. In size it approaches the presternum of 

 Inia, but there are no traces of conical processes behind the articular 

 surfaces for the first ribs. The anterior extremity is imperfectly 

 preserved and both angles are missing. The presternum is larger 

 than that of Platanista and smaller than that of Inia; the posterior 

 extremity is abruptly truncated. The breadth of the presternum 

 posteriorly (52 mm.) equals about one-half of the mesial length 

 (113 mm.). The external margins are rounded and the bone, as a 

 whole, is relatively thick (greatest thickness 24.5 mm.). The ar- 

 ticular surfaces for the second ribs are situated on the external face 

 and at the posterior end of the presternum, and not on the posterior 

 face as in Platanista. 



A pair of curved plate-like bones (pi, 11, fig. 2) which are united 

 on their antero-internal margins may represent the mesostemum. 

 Such a type of mesosternum would be somewhat unusual. The meso- 

 stemum of Platanista is composed of two flattened bones which are 

 in contact for their entire length. 



PADDLE BONES. 



Although the phalanges (pi. 9, fig. 4) have been arranged in 

 accordance with the position of similar phalanges in the paddle of 

 Inia geo-ffrensi-s., no assurance can be given that this arrangement is 

 correct. These bones were found intermingled with other parts of 

 the skeleton and may represent parts of both paddles. 



EXPLANATION OF PLATES. 



Zarhacliis flagellator Cope. Cat. No. 10485, Division of Vertebrate Palaeon- 

 tology, United States National Museum. Calvert formation, western shore of 

 Chesapeake Bay, about one-half mile south of Chesapeake Beach, Calvert 

 County, Maryland. Collected by Norman Boss, August 8, 1921. 



Plate 1. 



Skull of Zarhachis flagellator Cope. About % natural size. Upper figure. 

 Dorsal view ; Lower figure. Ventral view. The posterior end of the skull has 

 been i-estored. Abbreviations: Ex. pt., external pterygoid; Fo. inf., infraorbital 

 foramen ; Fr. frontal ; In pt., internal pterygoid ; Max., maxilla ; Max. cr., max- 

 illary crest ; A^ A., external nasal apei-ture ; Na., nasal ; Pmx., premaxilla ; Po. 

 gl. p., postglenoid process of squamosal ; Po, p., postorbital projection of frontal ; 

 Pr. p., preorbital projection of frontal ; Prs., presphenoid ; So., supraoccipital ; 

 S. or. pr., supraorbital process of frontal ; V., vomer ; Ziig., zygomatic process of 

 squamosal. 



