MARYLAND GEOLOGICAL SURVEY 5 



highly developed of the forms, anchylosed together in a short mass of 



bone, this leaves the animal with no apparent neck; in other forms the 



cervical vertebrae are all separate and the head and "body are separated 



by a well-defined constriction. All forms show a distortion of the 



anterior portion of the skull, which in some reaches a high degree. 



The suborder has four families: Squalodoxtidae, Platanistidae, 



Delphinidae, and Phy'Seteridae. 



Following is a scheme given by Cope ' for the determination of the 



various families: 



I. Teeth of two types, one and tAvo-rooted. 



Neck longer; teeth in both jaws Squalodontidae. 



II. Teeth uniformly one-rooted. 



a. Bibs nearly all two-headed. 



Teeth in both jaws; neck generally longer Plfitanistidae. 



Teeth in lower jaw only; neck short Physeteriihie. 



aa. Four or five anterior ribs only two-headed. 



Teeth in both jaws; neck short DelpMnUlae. 



In the same article he speaks of the characters here selected to 

 designate the different families. He says: "All the above characters 

 are those of divergence from the principal mammalian stem, and have 

 relations to the conditions of aquatic life. Thus the posterior position 

 of the nostrils permits inspiration without the elevation of the muzzle 

 above water level, which is rendered difficult, if not impossible in the 

 most specialized types, by reason of the extreme flatness and inflexi- 

 bOity of the cervical vertebrae. The absence of teeth is appropriate 

 to the habits of the types which lack them." (The confinement of the 

 diet of the Mysticoceti to soft bodied animals.) " The disarticulation 

 and the disappearance of the heads of the ribs in the Mysticoceti is 

 appropriate to the support which all the viscera derive from the fluid 

 medium in which these large animals live." Again, " The line of the 

 successional modification of the Cetacea is found in the changes in (1) 

 the shape of the skull; (2) the extinction of the dentition; (3) the 

 shortening of the cervical vertebrae; and (4) in the separation of the 

 ribs from articulation with the vertebral centra. The modification of 

 the shape of the skull is related to the gradual transfer of the external 

 nostrils to more and more posterior positions." 



1 Amer. Nat., voL xxiv, 1890, p. 60'i. 



