34 CALIFORNIA ACADEMY OF SCIENCES 



Thalattosaurus alexandrae 3Ierr>am. 



Plates I, II; Plate IV, Vu:. 1 ; Plate V, Figs, lfi-2/,; Plate VI; Plate VII, Figs. 1-.3, 8-9; 



Plate VIII, Figs. 1-2, -ta-6. 



Bull. Drpt. Ccdl. Univ. Calif, v. 3, no. 21, p. 419. 



Type spediiK'n No. 9085 Univ. Calif. Palae. Coll. 



Tliis is tlie largest species of tlu' group. The known siieeinicns appear to 

 represent individuals attaining a length not far from two metres. Neural spines 

 of vertebrae not greatlv widened. Propodial and epipodial limb elements and 

 scapula considerably expanded. Nasal openings long, portion of nasal elements 

 behind the nares relatively short. Slender anterior teeth with widely' spaced striae. 

 Posterior mandibular teeth ver\' low-crowned, l)utton-like. The number of teeth 

 on the dentary appears to have been eighteen. 



A large part of our knowledge of the Thalattosauria has been obtained 

 through the study of this species, of which several representative specimens are 

 known. All of the material representing this form has been obtained in the 

 Trachyceras horizon of the Hosselkus Limestone. No occurrences are known 

 outside the Shasta region of California. 



MEASUREMENT.S. 



Vertebrae. 



mm. 



Anterior dorsal (No. 9084), height of centrum 20 



width " " 23 



length" " 16 



" " " height of upper arch 46 



" " " antero-posterior diameter of upper arch at middle 



height of spine 13 



" " " transverse diameter of uj)per arch at middle height of 



spine V(5 



Anterior caudal (No. 9085), height of centrum 30 



" " " width " " 13 



length" " 18 



" " " height of upper arch 48 



" " " antero-posterior diameter of upper arch at middle 



height of spine 8 , 



" " " transverse diameter of upper arch at middle height 



of spine 6 



la, approximate. 



