556 F. B. Loomis — New Mosasaur from the Ft. Pierre. 



Elasmobranchii Corax represented by 2 teeth and three strings of 



vertebrae. 

 Teleostomi Portheus " " one pair of lower jaws. 



Ichthyodectes " "18 strings of vertebra?, by 



portions of the tail, and by 

 the dentale and maxilla. 

 Saurocephalus " " 2 crushed skulls. 



Empo " " skull bones and vertebrae. 



Pachyrhizodus " " vertebrae. 



Reptilia Platecarpus " " many parts of skeletons. 



Elasmosaurus " " 1 paddle and part of a 



second. 



The mosasaurs were far the most abundant fossil and the 

 best preserved. We found one skeleton with a complete skull 

 followed by 23 vertebrae and their ribs all in place, a second 

 consisting of a disassociated skull, 7 anterior vertebrae and a 

 scattered front paddle, a third consisting of parts of the skull 

 and 56 vertebrae from the mid-body to the middle of the tail, 

 another consisting of the front paddle practically all the 

 phalanges being in place, and a dozen less important speci- 

 mens, one a considerable part of the posterior paddle. These 

 are the basis of the new species to be described. They were 

 all found within 100 yards of each other not varying in horizon 

 more than 10 or 15 feet. 



Platecarpus brachycephalus sp. nov. 



I take as the type the disassociated skull, No. 389 in the 

 Amherst Collection, and as acotype the complete skull No. 398. 

 To what these give, I add data and drawing from some of the 

 other material from the same place and level. 



The first question raised is as to the genus of the form. The 

 following table gives the most salient features by which the 

 mosasaur genera are subdivided. 



In the brevity and width of the skull, the small number of 

 teeth in the maxilla, the Amherst specimens resemble Brachy- 

 saurus, but in the shape of the humerus, in the fact that the 

 suprastapedial process of the quadrate is not coosified with the 

 base of the quadrate behind, and in the fact that the chevrons 

 of the caudal vertabree are free and articulated to the centra, 

 it resembles Platecarpus. In the major and in a number of 

 minor features the form is intermediate in character between 

 the Ft. Pierre genus Braclrysaurus and the Niobrara genus 

 Platecarpus, but I consider it nearer to Platecarpus as its dif- 

 ferences from Platecarpus are mostly in degree, w T hile those 

 from Brachysaurus are positive ones and mark a wider diver- 



