242 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



its way through the mud and soft sands in a maimer similar to that 

 of Limuliis is proven by the trails and burrows left by it which we 

 now designate as Cruziana [Walcott, 1891, pis. 64-66]. 



Facial sutures. — The facial sutures are rarely represented, even 

 by elevated lines on the exterior surface or depressed lines on the 

 interior surface of the cephalon. If we accept Beecher's view that 

 the sutures are in a condition of symphysis [Beecher, 1897, p. 191], 

 and that the elevated and depressed lines represent the suture be- 

 tween the cranidium and free cheeks, the latter bear the visual 

 surface of the eye. In my hurried study of the Olenellus fauna in 

 1886 and 1 89 1 I permitted facial suture lines to be represented in 

 front of the eye in a specimen referred to Olenellus gilberti [Wal- 

 cott, 1886, pi. 20, fig. ih; 1891, pi. 86, fig. ili] on evidence that now 

 appears to me to be insufficient, as the line may have been formed 

 by a fracture in the test. 



In one specimen of Pcedeumias trausifans [pi. 33, fig. i] an ele- 

 vated line having the usual curvature of the posterior facial suture 

 starts from the base of the eye at its posterior third and extends 

 with a gentle sigmoid curve outward and backward to the postero- 

 lateral angle of the cheek where it fades away. It is not probable 

 that this line represents the facial suture that has been lost in the 

 development of the cephalon, but it suggests that conclusion. 



Prof. R. P. Whitfield [1884, p. 151, pi. 15, fig. i] describes and 

 illustrates the curve of facial sutures in Olenellus tJiompsoni.^ The 

 curve assigned to the sutures back of the eye is certainly incorrect, 

 as, from many specimens, we know that the elevated lines run to 

 the intergenal angles, and I strongly suspect tliat the suture as out- 

 lined in front of the eye is based on a crack in the test, as the speci- 

 men is flattened in the arenaceous shale. 



Anterior glabellar lobe. — The anterior or first lobe of the glabella 

 in the young stages of growth is small and a part of the palpebral 

 segment of the cephalon [pi. 25, figs. 9, 10, and 22]. In what I con- 

 sider to be the most primitive genus of the Mesonacidae, Nevadia 

 [pi. 23], the first lobe is small and not at all expanded as in Oloiellus 

 [pl- 37]- I^ Callavia [pis. 27 and 28] the first lobe is also small, 

 although the genus occurs at a much higher horizon than Nevadia. 

 We find that Holmia weeksi [pi. 29] which is associated with 

 Nez'adia has an expanded first glabellar lobe. That the small, 

 contracted first lobe of Nevadia is a primitive character is shown by 

 its occurring in the youngest stages of growth of all the genera of 



^ Referred in this paper to Pcedeumias transitaiis. 



