* 



Vol. 5] Merriam. — A Primitive IcMhyosaurian. 383 



Dentition differentiated ; posterior teeth laterally-compressed, 

 low-crowned ; most anterior teeth known relatively small, high- 

 conical, nearly circular in cross-section. Teeth inserted in pits 

 which may be situated at the bottom of a shallow groove. Roots 

 showing coarse longitudinal folds, with little or no cement 

 covering. 



DENTITION. 



The first specimen to be obtained (no. 9924) consisted of 

 portions of two lower jaws; and a part of the skull, probably 

 representing the maxillary region, containing two nearly parallel 

 rows of teeth quite similar to those of the lower jaws ( pi. 40. 

 fig. 2). Very little of the skeletal structure could be determined 

 as the bones were very fragmentary. The elements of the 

 dentition were well preserved. 



Pig. 1. — Phalarodon fraasi. Inferior dentition. No. 9924, natural size. 



The teeth situated on the mandibular elements of specimen 

 9924 (fig. 1) are of a thick, low-crowned type. They are sep- 

 arated from each other by bony partitions, and appear to be 

 in completely enclosed pits. There is no evidence of the pres- 

 ence of cement surrounding them. Only the thinnest possible 

 cement layer could have been present, as the enclosing bony 

 tissue of the dentary surrounds the roots very closely. The 

 roots are much compressed laterally, and the walls show strong- 

 vertical folds. The teeth vary greatly in size and form accord- 

 ing to their position in the jaw. The crowns of the most 

 posterior teeth are very low and are considerably compressed 

 laterally. The summits of the crowns are quite abruptly 

 rounded. The most anterior mandibular tooth is hardly more 

 than one-quarter the size of the next to the last one in the series. 

 Its crown is almost round in cross-section, and is more slender 



