GEOLOGICAL SURVEY OF THE TERRITORIES. 399 



iscliia is strongly deflexed behind the transverse rib, and is continuous 

 with the basin-like concavity formed by the united pubes. The glenoid 

 surface of the pubes is a sigmoid, while that of the ischia is regularly 

 convex. The articulation of the ilium has been exclusively with the 

 former. 



Of the pleurapophysial portion of the two arches nothing appears to 

 be preserved except two lateral, symmetrical, long bones. One was 

 found imbedded in the mass carrying the pelvic arch, and they articu- 

 late well with the pubes j but the articular extremity is too short to 

 articulate with ischia at the same time. Though they resemble the 

 inferior view of the procoracoids, they represent the ilia of Plesiosaurus. 

 The head is subdiscoid, rather flat, slightly projecting eccentrically 

 with a ligamentous pit. The articular surface is very oblique to the 

 axis of the shaft, and is separated from the surface by a marked angle 

 all around. Nothing like a trochanteric ridge is apparent in this bone. 



In. Lin. 



Length in middle of curve 9 9 



Diameter at head 3 3 



Diameter distally on curve 6 



Diameter distally straight 4 



The shaft is flattened cylindric; much flattened nearest the proximal 

 extremity. The latter is very oblique to the shaft and slightly convex 

 near the proximal margin. 



The end of tlw muzzle preserved includes also the symphysis and parts 

 of the rami of the mandible. The parts have been crushed together, 

 and the ends of the teeth broken: off. The alveoli of the two jaws 

 incline at a narrow angle to each other ; hence the teeth, which alternate, 

 cross each other near the middles of the crowns. The parts preserved 

 appear to belong to the prem axillary bone, though no suture can be 

 found and the bony walls are so thin as to render their obliteration a 

 probability. There is a keeled ridge along the middle line above, which 

 is not continued to the margin of the bone. The form of the muzzle is 

 narrow, the sides subparallel near the tip, which is elongate rounded. 

 The mandibular symphysis, however, is not very elongate, as the rami 

 are given off at three inches from the tip. The latter appear to have 

 been quite slender from the various small sections or pieces sent with 

 the muzzle. The premaxillary border of 4 in. 7 lin. exhibits eight teeth, 

 or their alveoli, of which the median two are close together, and not 

 separated by any mandibulars. The sections of the teeth are round or 

 oval, and their sizes are irregular, probably on account of differing age 

 and degree of protrusion. The diameters at alveolar margin vary from 

 6 lines to 3. Their form is slender conic, or with the root slender fusi- 

 form, and the pulp cavity is small and median, sometimes cylindric, and 

 sometimes narrowed. The surface, from a short distance above the 

 alveolar margins to the tip, is marked with acute, thread-like ridges, 

 which are sometimes interrupted, and sometimes furnished with short 

 branchlets. They are more or less undulate, and do not unite, but sim- 

 ply cease as the tip of the tooth is approached. The latter is smooth 

 without lateral cutting edges. The width of the mandible at the com- 

 mencement of the rami is 3 in. .05 lin. ; of the muzzle of the seventh 

 tooth 3 in. 7.5 lin. j at the third tooth 2 in. 4.2 lin. 



General Remarks. — The tail is a powerful swimming organ, more or 

 less compressed in life ; hence the specific name, which means flat-tailed. 



The danger of injury to which such an excessively elongate neck has 

 been exposed, would render the recovery of a perfect specimen like the 



