86 University of California Publications in Geology [Vol. 13 



6. The femur is very short in Lutra and Phoca; it is considerably longer 



relatively in Otaria and Ursus. 



7. In Lutra and Enhydra the floor of the orbit formed by the maxilla is very 



large, as it is also in Leptonyx, at least, amongst the Phocidae, while in 

 others of that family it is of moderate size. It is very small in Otaria 

 and Trichedhus, as it also is in Ursus. 



8. There are noteworthy defects in the ossification in the cranial walls in Lutra 



and the Phocidae. There are no such defects in Ursus or Trichechus, 

 while they are but of small extent in Otaria. 



9. The suborbital foramen is very large in Lutra and Phoca barbata and 



Trichechus. It is small in the Bears, and of moderate size in most 

 Otaries. 



In support of this theory there are certain structural peculiarities 

 in the early Lutrinae which are certainly worthy of consideration. 

 Potamotherium valetoni, of the famous lacustrine beds of St. Gerand- 

 le-Puy, Department of Allier, France, presents many characters which 

 may indicate relationship with the Phocidae. There is a striking simi- 

 larity in certain peculiarities and proportions of the humerus, radius, 

 ulna, femur, and tibia of Potamotherium valetoni with those of Phoca 

 richardii, the resemblance being even closer with the Miocene form, 

 Phoca vindobonensis. The teeth of Phoca gichigensis may as readily 

 be derived from a pattern like Potamotherium valetoni as from a pat- 

 tern like Cynarctus. A comparison of the humerus of Potamotherium 

 with one of Phoca shows that an entepicondylar foramen, an expanded 

 and flaring supinator ridge, and a prominent deltoid ridge are present 

 in both. In Potamotherium the internal tuberosity is rudimentary 

 while it is greatly produced in Phoca. On the other hand the external 

 tuberosity is produced in Potamotherium though it is wanting in 

 Phoca. The following differences may be pointed out in the femur. 

 There is present a rounded depression in the head of the femur of 

 Potamotherium for insertion of ligamentum teres, which is absent, 

 however, in Phoca and Laiax. The shaft of the femur is relatively 

 longer and slenderer, the neck is longer, and the head proportionately 

 smaller than in Phoca. The lesser trochanter is wanting in Phoca 

 though it is present in P 'otamothcrium. 



The writer does not wish to convey the impression that Potamo- 

 therium might itself be considered the ancestor of the Phocidae. If 

 the phocids are derivatives of this stock, then it is probable that one 

 of the forbears of Potamotherium was the source and that the 

 Lutrinae and the Phocidae are both descendants of that type. In this 

 connection it should be pointed out that Andrews 174 figured and 



174 Andrews, C. W., Descriptive catalogue Tertiary vertebrata of the Fayum, 

 Egypt. Publ. Brit. Mus., pp. 218, 229-230, text fig. 74. London, 1906. 



