OLENELLUS AND OTHER GENERA OF MESONACID^ 295 



Dimensions. — A dorsal shield 44.75 mm. in length has the follow- 

 ing dimensions : 

 Cephalon: ,„„j_ 



Length 16. 



Length of glabella 13. 



Length of eyelobe 6.5 



Width at posterior margin 30. 



Width of glabella at posterior margin 8. 



Width of glabella at broadest part of anterior lobe 10. 



Thorax : 



Length 26. 



Width at first segment 23. 



Width of axial lobe at first segment 9. 



Width of axial lobe at last segment 2.5 



Width of pleural lobe at first segment 7. 



Pygidimn: 



Length 2.75 



Width 2.5 



The preceding" description is based on adult specimens from 40 mm. 

 to 50 mm. in length. The largest cephalon in the collection has a 

 length of 30 mm. ; width 50 mm. This indicates a dorsal shield 

 of about 85 mm. in length. One young stage of growth is partly 

 illustrated by a broken cephalon 1.5 mm. in length in which the 

 outline [fig. 9] is rounded; glabella cylindrical, with the base of the 

 frontal lobe continuous with the strong palpebral segment ; this may 

 be compared with the stage of EUiptoccpJiala asaplwidcs represented 

 by fig. 14 of pi. 25. 



Observations. — Fragments of this species occur abundantly in asso- 

 ciation with Nevadia wceksi in hard, shaly sandstones deep down in 

 the Lower Cambrian (Georgian) section of southwestern Nevada. 

 In America it is the oldest species of the genus Hoiinia, and in de- 

 velopment its position in the Mesonacidas appears to be after the 

 forms with imperfectly developed posterior thoracic segments : A'e~ 

 vadia, Mcsonacis, and the slightly more specialized Callazna. 



The generic relations of Holinia rozcci to the genotype H. kjenilH 

 are very close as may be seen by comparing the illustrations on pi. 

 29 with fig. 7, pi. ly. The eye lobes are of the same character, as 

 are the other parts of the cephalon ; each species has sixteen thoracic 

 segments that terminate in narrow arching spines ; the pygidia are 

 small and of the same type. The occipital spine of H. rozvei is 

 longer than that of H. kjcrulfi. The specific differences are in the 

 details of the cephalon, such as the relation of the anterior lobe of 

 the glabella to the frontal border ; the longer occipital and genal 



