558 S. W. WILLISTON 



The teeth in Mycterosaurus are characteristically different from 

 those of VaranO'pSy Ophicaodon, Dimetrodon, or Edaphosaurus. 

 Altogether there are about eighteen, possibly one more, on each 

 maxilla, the first four or five the largest. They are moderately 

 elongated and flattened, with a rather obtuse apex. 



Of the skeleton of this specimen, there are fragmentary verte- 

 brae, and a coraco-scapula. The posterior coracoid is well ossified 

 and not distinguishable from the anterior. It has a straight line 

 below and is rather narrow. The interclavicle appears to be of the 

 usual form, and very unlike the extraordinary interclavicle of 

 Pantylus. The vertebrae, so far as they are preserved and pre- 

 pared, are like those of Varanops. The spine is thin, and not more 

 than two or three times the height of the centra. 



Relationships. — There can scarcely be dissent from the opinion 

 that Mycterosaurus is related to Dimetrodon, and should find its 

 natural place in the same suborder, the Pelycosauria, though in a 

 different family. The skull and vertebrae are more primitive, but 

 both have the same relations of the bones, the same temporal 

 opening, and the same elongated, narrow skull. The lower jaw lacks 

 the inflected angular process below, but that is a specialization that 

 would hardly be expected in the more primitive form. I am aware 

 that this peculiarity has been largely relied upon as indicative of the 

 relationships between the American and African Theromorpha, 

 but I am skeptical of its value. No other American genus shows 

 it, except, in a lesser degree, Sphenacodon and Edaphosaurus, and 

 these are all highly specialized animals with elongated spines. 



The family determination of Mycterosaurus, until more of the 

 skeleton is known, is doubtful; it may provisionally be placed in 

 the Poliosauridae. Of the American reptiles, nearly every speci- 

 men that can be differentiated by decisive characters represents a 

 distinct genus; and the majority of genera, when fully known, are 

 more or less justly placed in distinct families. The following 

 genera, it seems to me, are rightly separated in different families; 

 a fuller knowledge may require further division. 



Sphenacodontidae 



Clepsydrops Cope 

 Sphenacodon Marsh 

 Dimetrodon Cope 



