36 - oaltfornia academy of srie>m'ks 



Measurements. 



mm. 



HuniiTUs (No. 9120), length 32 



Ulna " " 21 



greatest width 11 



Thalattosaurus perrini, n. s]). 



Plate IV, Fiu. 3; Plate V, Fig. 3; Plate VII, Fi(i. 6. 



Posterior mandibular teeth with hiterally compressed and obtuse but not 

 greatly depresssed crowns, grading anteriorly into a slender conical form on the 

 anterior portion of the jaw. Maxillary teeth slender conical, set in deep pits. 



The dentition of the dentary and the maxillary are well shown. The den- 

 tarv holds an interrupted series of twelve teeth with spaces for several more 

 among them. The entire number lias probably been sixteen to eighteen. The 

 posterior ones have low, broad, laterally compressed crowns. Tlic individuals in 

 the middle of the series were short, broad, and leaf-like, their margins being verj' 

 close together. The anterior members of the series have a shorter antero-posterior 

 dianreter, higher crowns, and a slender conical form without lateral compression. 

 The most anterior tooth, seen only in oblique section, shows strong longi- 

 tudinal striations or ridges on its surface. The most posterior teeth are close 

 together and in shallow pits. The anterior ones are deeply set and are more 

 widely spaced. 



On the maxillary there are five teeth of slender conical form, witli spaces for 

 one or two more. They are in deep, distinct pits. 



As much of the vomerine dentition as is known is very similar to that of 

 Tha laftosauru s a lexandrae. 



The general characters of this form are tliose of a typical Thalattosaurian. 



The type specimen, the only known sjiecimcn of this species, was the first 

 Thalattosaurian discovered. It was found by Professor James Perrin Smith in 

 the Trachyceras beds of tlie Hosselkus Limestone at Smith Cove, near Scjuaw 

 Creek, Shasta County. There was exposed on the slab only a prevomer with its 

 peculiar, bhmt teeth. Later the portion of tlie skull sliown on Plate iv, figure 3 

 was exposed by preparation witli steel points and l)v etching with very dilute 

 hydrochloric acid. After the jaws had been partly uncovered, the writer pub- 

 lished a note on the Triassic Reptilia from Northern California,^ in which this 

 specimen was referred to as a Shasfasaurus with a heterodont dentition. Later 

 it was discovered that it could not belong to the Ichthyosauria, but the true 

 affinities were not known until after the discovery and study of the type of 

 Thalattosaurus alexandrae. 



I Science, n. ser. t. 15, p. 411. 



