136 PATAGONIAN EXPEDITIONS! PALAEONTOLOGY. 



narrow and elongate, while the ventral border of the arch has a gentle 

 slope downward and forward ; the suborbital process of the jugal and 

 maxillary is rather short and has an almost vertical direction ; /. e., at 

 right angles to the plane of the upper teeth ; the jugal has hardly any 

 trace of a postorbital process, but that of the frontal is quite distinct. The 

 occipital condyles are usually almost sessile and project downward rather 

 than backward, though there is considerable variation in this respect. 

 The infraorbital canal is sometimes single and sometimes double, a differ- 

 ence which appears to be entirely individual. 



The great majority of the skulls of this species which have been col- 

 lected have suffered more or less distortion from pressure and they often 

 have a very different appearance in accordance with the direction in which 

 this pressure was applied, whether it was vertical, horizontal, or oblique. 

 The bones of the skeleton, which have been described in the account of 

 the genus, appear to be almost exactly alike in the two species. Whether 

 any constant difference occurs in the structure of the exoskeleton, is still 

 uncertain, for the number of fairly complete carapaces which have been 

 collected is still too small to show whether the remarkable differences be- 

 tween them are specific or merely individual. 



The following measurements were made from No. 15,212 in the Prince- 

 ton collection ; this is a fine specimen, with most of the carapace and a 

 large part of the skeleton, which agrees closely in size with the La Plata 

 skeleton. It was collected by Mr. Hatcher at Corriken Aike. 



MEASUREMENTS. 



Upper dentition, length 094 S-, length 0105 



1, length (t. c., antero-posterior diameter) .003 " width 007 



" width (/. e., transverse diameter) 0035 Skull, length in median basal line 144 



, length 0055 " extreme length 160 



" width 004 Cranium, length to anterior rim of orbit . 1 10 



*, length 009 Occiput, height 041 



" width 004 " width at base 064 



A, length 01 1 Zygoma, length 080 



" width 0045 " length of suborbital process . . .046 



, length 012 Skull, width over suborbital processes. . .100 



" width 006 Palate, length in median line 104 



*-, length 0125 " width at i 018 



" width 007 " " " * 024 



i, length 0125 Rostrum, length 040 



" width 007 " width at infraorbital foramen . .048 



