114 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



All the available specimens are either of such old individuals, or have 

 suffered so much from crushing, that it is exceedingly difficult to make out 

 the various bones of the skull, and some I have not been able to determine 

 at all. The basioccipital is moderately elongate, broad and with a ventral 

 keel, the prominence of which varies in different individuals, becoming 

 higher with age : on each side of the bone is a conspicuous pit, or rugosity, 

 for the attachment of the rectus capitis muscle. Even in very young skulls, 

 the basi- and exoccipitals are indistinguishably fused together, without 

 any trace of a suture. The exoccipitals are low, broad, extremely thick 

 and massive, and so dense that the cancellous structure is not immediately 

 obvious ; the paroccipital processes are very conspicuous and of moderate 

 length, descending below the level of the condyles, and are broad and 

 antero-posteriorly compressed, with convex anterior, and concave posterior 

 surface. The condyles are quite large and the articular surface of each is 

 prolonged forward upon the basioccipital in much the same way as in 

 Macrauchenia. Large condylar foramina perforate the basioccipital later- 

 ally and are entirely concealed by the overhanging condyles, when the 

 skull is viewed from below. The supraoccipital is a small, but very thick 

 and heavy bone, massive and dense and without a sinus. 



The basisphenoid is broad and heavy, contracting forward. The limits 

 of the ali- and orbitosphenoids in the cranial wall cannot be definitely 

 made out in any of the specimens at my disposal. The parietals are very 

 large and form the greater part of the brain-case, which is narrow and 

 of no great capacity. Anteriorly, the parietals are deeply emarginated in 

 the median line, to receive the broad posterior processes of the frontals. 

 The sagittal crest is broad anteriorly, contracting posteriorly to a thin 

 ridge, the length and height of which are variable, though in general they 

 may be said to increase with age. There are also marked specific differ- 

 ences in the development of the sagittal crest, as, for example, between 

 T. lallenianti and T. garrettoriim. 



The squamosal is long and low, not rising much upon the side of the 

 cranium, most of which is formed by the parietal ; the posttympanic process ■ 

 is short and inconspicuous and is closely applied to the mastoid. The 

 zygomatic process is quite long, extending forward to form a small portion 

 of the lower margin of the orbit, but has no trace of a postorbital process ; 

 it is slender and laterally compressed, and arches out but little from the 

 side of the cranium ; the proximal portion, or root, of the process is much 



