THALASSEMYDID&. 135 
progress with the description. “That which especially strikes one is the greater thickness of 
the bones here described, as compared with corresponding ones of O. emarginatus and O. gibbr. 
The three types are of nearly the same size. The dimensions of various bones are given in 
the following table. Except the length, the dimensions of each bone are taken at the anterior 
end. The thickness of the nuchal is taken 
at the midline. The columns headed by 
Element. Length. Height. Thickness. : J 
1 belong to O. emarginatus; those by 2, 
Pe etal eel ala | 2 | 4 | te Ougrbde: those by 3, to O.robustus. 
The nuchal bone (fig. 147) appears 
| Nuchal .... | 122 | 127 | 125 vss js. | T§ 16 20 to have extended backward further in 
Peripheral 1 = 63 79 78 | 48 60 60 19 17 30 : : : : : “os 
5 acalleraee levee Ve ea lie i >» this than in the species with which it is 
2 82} 83) 4 50 | 60 ZO: | 21 | 26 : : 
Io go 89 111 110 14 18 compared above. The hinder portion 
u 83 90 110 |Loo+ '9 of the bone is broken away, but the por- 
Pygal oes 83 96 71 | 68 20 / 
tion remaining is 80 mm. wide antero- 
posteriorly. The nuchal of O. emargina- 
tus measured in the same place gives us 70 mm.; the nuchal of O. gibbi has a length of 75 mm. 
Fig. 148 is a section at the articulation between the nuchal and the first peripheral. 
The upper border of the first peripheral, articulating with the first costal, has a thickness 
of 10 mm. (fig. 148). The left second peripheral lacks the hinder end; and that of the right 
side has the hinder end of the inner face so eroded that it does not show the pit for the hyo- 
plastron. These peripherals articulated with the first costal. The other peripherals are 
missing to the ninth, and of these only unimportant fragments remain. The tenth and eleventh 
(fig. 149) are nearly plane above, or slightly convex. The rib-pits are flat; and that of the 
we 
Fic. 147.—Osteopygts robustus. Nuchal bone and first peripheral right and left, of type. 
nu. s, nuchal scute; per. 1, first peripheral. 
tenth is rooft over to the articulation with the eighth costal plate with bone 7 mm. thick. It is 
possible, but not probable, that the ninth peripheral had its upper border notcht by the rib- 
pit, as it is in the case of all the hinder peripherals of O. grbbr. No rib entered the eleventh 
peripheral. This and the pygal articulated suturally with the last suprapygal. The upper 
sutural border of the pygal is 13 mm. thick. The free borders of the tenth and eleventh 
peripherals are acute. Fig. 150 represents the front end of the tenth peripheral. There are no 
indications of emarginations of the free border such as are supposed to characterize O. emar- 
ginatus. The pygal (fig. 149) is notcht somewhat at the midline. Fig. 151 is a section thru 
the middle of the pygal. 
There are present what are regarded as the proximal and the distal portions of the left 
fourth costal; the proximal end of the right fourth; the left seventh, lacking most of the front 
border; the left eighth entire; and the proximal end of the right eighth. These bones are all 
thick. Where the suture with the neurals is shown the bone is 10 mm. or If mm. thick. At 
the middle of the length of the costals the sutural edges are about 6 mm. thick. The distal 
ends of costals four and five were suturally joined to the corresponding peripherals; and the 
same is true of the seventh and the eighth. The fourth costal has a width of 62 mm. at the 
costo-vertebral sulcus and 70 mm. at the distal end. The end of the rib of this costal projects 
from near the front border of the distal end, as it appears to do, likewise, in O. gibb:. The width 
