104 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. rol.YI. 



pneumatic foramina are seen at or near the usual localities. The largest 

 foramina for the exit of the roots of any pair of sacral nerves is gen- 

 erally in the fifth vertebra; they decrease in size as they leave them 

 either way. In the young only the last few of these foramina are double; 

 they are all double in the adult and placed one above another, a i)air on 

 the side of each centrum at their posterior borders, for the exit of the roots 

 of the sacral nerves. The diapophyses of the anterior five sacral verte- 

 brae are thrown out against the internal surfaces of the ilia, to which 

 they are firmly attached, and act as braces to hold the engaged bones 

 together. The parapophyses of the first form facets for articulation 

 with the sacral ribs; the second and third have none; in the fourth and 

 fifth they also act as braces in the manner above described, joining the 

 ilia just before their divergence commences. Eeliance seems to have 

 been placed eutirely in the completeness of the sacro-iliac union in the 

 last vertebrge, for the apophysial struts terminate in that portion of the 

 pelvic vault formed by the sacrum itself, except in the last two verte- 

 brae, where the i)arapophyses abut against the iliac borders. The para- 

 pophyses of that vertebra which is opposite the acetabula are promi- 

 nent, they being long and ample, reaching to the border and reenforcing 

 that part of the pelvis that requires it the most, the vicinity of the 

 leverage for the i^elvic limbs. In other Sfrigidce several apophyses are 

 thrown out at this point. The posterior opening of the neural canal in 

 the last sacral vertebra is subcircular, its diameters being about a milli- 

 metre in length. This vertebra also possesses small postzygapophyses, 

 looking upwards and outwards for articulation with the prezygapophyses 

 of the first coccygeal vertebra ; the articulating facet of the centrum is 

 also small, long transversely, notched in the median line, the surface on 

 either side being convex. At every point where the sacrum meets the 

 iliac bones union is firm and complete, though both upon the internal 

 and external surfaces the sutural traces are permanently apx^arent. 

 The anterior iliac margins, as tliej^ diverge from the sacral spine, form an 

 acute angle, concave forwards ; they have a well-marked rim or border, 

 ne'arly a millimetre in width, raised above the general surface of the 

 bone, which disappears on the outer borders as we follow them backwards. 

 The two anterior and outer angles overhang the sacral and fifth or 

 last dorsal pleurapophyses. From these last the marginal boundaries, 

 which necessarily give the bones their form, are produced backwards and 

 outwards to a point opposite the centrum of the third sacral vertebra, 

 then backwards and inwards, forming at the above points two lateral 

 angles. From the apices of the two lateral angles to where the borders 

 terminate on either side in front of the acetabula with the pubic bones, 

 the direction is such as to form a concavity on each side; the line joining 

 the bases of these concavities, points opposite the posterior openings of 

 the ilio-neural canals, being the narrowest part of the pelvis. The upper 

 and at the same time the inner margins of the bones in question, from 

 the anterior and median angle, at first approach, soon to diverge from 



