MARYLAND GEOLOGICAL SURVEY 57 



vertical diameter of the former is nine inches and a half; the trans- 

 verse diameter of the former is nine inches and a half; the transverse 

 diameter eleven inches." There is no further description and there 

 are no figures. 



In the collection of the Maryland Geological Survey there is a radius 

 and ulna of what was evidently a small species of Tricliechus, and a single 

 isolated rib has much the appearance of belonging to the same genus. 



Occurrence. — Calvert Formation. Fairhaven. 



Collection. — Maryland Geological Survey. 



With the earliest Miocene the Odontoceti appear in considerable 

 numbers, all the families and many of the genera being found in de- 

 posits of that age. The Mysticoceti appeared only later and reached 

 their highest development in the Pleiocene and the recent. 



The ancestry of the Sirena is as little known as that of the Cetacea 

 but the earliest known form has not reached quite such a high state 

 of specialization as the earliest Cetacea. In Prorastomvs from the 

 Eocene of the Island of Jamaica, the single genus of the family Pro- 

 rastomidae, the teeth are of the angulate type and are present in 

 complete series in both the upper and the lower jaws. In the Mana- 

 tidae the incisor and canine teeth are wanting and the cheek teeth are 

 similar, while in the Halicoridae there is a large pair of incisor teeth 

 which may be absent. In outward form the Sirenia ha^^e taken on 

 the fish-like form which is best adapted to the environment but there 

 remains an imperfectly developed pelvis and the rudiments of the 

 femoral bone. 



The origin of the Cetacea and of the other aquatic order, the 

 Sirenia remains one of the most puzzling questions of phylogeny. To 

 the first is generally accorded an origin from some primitive and gen- 

 eralized member of the carnivorous stem, and to the latter an equally 

 obscure origin from the ungulates. The necessary connecting links to 

 prove these suppositions are, however, sadly lacking. Beddard, in his 

 recent book on Whales, would accord to the Cetacea an ungulate 

 ancestry, but this he does with much diffidence. He says (page 99) : 

 " It is to be feared that nothing can be done except, and that vaguely, 

 to suggest an Ungulate-like ancestor." Albrecht would assign to the 



