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University of California Publications. r [Geology 



upper division. The limestone of this horizon is composed in a 

 large part of broken shell fragments, and the bones are almost 

 invariably isolated, indicating that the deposit was formed in 

 rather shallow water, where the skeletons were tossed about by 

 the waves. At this horizon the remains are principally those of 

 Nectosaurus, Thalattosaurus bones being comparatively rare. 



Skull and Dentition. 



There are in the collections several specimens representing the 

 upper and lower jaws, and clearly showing the characters of the 

 dentition. Specimen 10753 (pi. 17. fig. 1) shows nearly all of the 

 maxillary excepting the anterior end. The posterior portion of 

 the bone exhibits the area of contact with the lachrymal or pre- 

 frontal, and the long posterior extension which was in contact 

 with the jngal. The teeth in this jaw are slender conical, with 

 the crowns showing a slight lateral compression. The structure 

 of this jaw differs considerably from that of a specimen (No. 

 10620) previously referred to tentatively as the maxillary of 

 Nectosaurus.' 2 The characters of specimen No. 10753 seem, how- 

 ever, to be pretty certainly those of the maxillary, and approach 

 those of the better known Thalattosaurus perrini. Specimen 

 10620 may therefore represent one of the other dentigerous 

 elements, though its reference is still uncertain. 



A small fragment consisting of two closely united dentigerous 

 bones (No. 10778) may represent the anterior ends of the pre- 

 maxillaries or the anterior portion of the prevomers. A single 

 perfect tooth near the anterior end of this fragment is short, 

 thick, conical, and somewhat more acuminate than the most an- 

 terior teeth of the prevomers in Thalattosaurus, but much less 

 slender than the anterior teeth of the jaws. 



A number of small specimens representing the lower jaw with 

 dentition show the teeth to be conical but apparently not as slen- 

 der as in the maxillary. The crowns also show more lateral com- 

 pression than was suspected in the study of the type specimen of 

 Nectosaurus. 



In specimen 10797 the greater part of a mandible is preserved, 

 but the coronoid element and the dentition are unfortunately 



^ Merriam, J. C, Op. eit. pi. 5, fig. 6. 



