236 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Family MESONACIDAE ' Walcott 



Mcsonacidce Walcott, 1891, Tenth Ann. Rept. U. S. Geol. Survey, p. 635. 

 (Cites OlencUus {Mesonacis) vcrmontaim as typical of the family. 

 Declines to propose term Olencllidcc as it was too much like the family 

 name Olenidcc.) 



Olenellidce Vogdes, 1893, Occasional Papers California Acad. Sci., 4, p. 

 254. (Cites Olenelhis thompsoni Hall as the type.) 



Olenellidce Moberg, 1899, Geol. Foren. i Stockholm Forhandl., Bd. 21, 

 Hafte 4, p. 316. (The author includes under this family name the 

 following species: Georgiclhis (Elliptocephala) asaphoidcs (Em- 

 mons), Olenellus thompsoni Hall, Holmia kjerulfi (Linnarsson), 

 Mesonacis vermontana (Hall), Mesonacis mickzvitzi (Schmidt), and 

 Olenelloides annatus Peach.) 



Olencllidcc Lindstrom, 1901, Kongl. Svenska Vet.-Akad. Handlingar, Vol. 

 34, No. 8, p. 12. (Uses Olencllida: as a group name for the " Olencllidcc 

 proper" and the Paradoxince.) 



Description. — Cephalon very large, wider than long, genal angles 

 with spines ; intergenal spines developed in young and may be pres- 

 ent in adult. Facial suture rudimentary, .or in a condition of syn- 

 thesis. Eyes crescentic or semicircular and attached more or less 

 closely to the anterior lobe of the glabella by a rounded ridge ; visual 

 surface of eyes with facets arranged in quincunx order. Hypostoma 

 usually with more or less spinose posterior margin. Thorax long, 

 composed of from 13 to 27 free segments. Pygidium siuall, margin 

 usually entire but may have from one to three spines. Surface of 

 test in adult specimens granular and usually with network of very 

 fine thread-like raised inosculating ridges. 



The genera included are Ncvadia, Mesonacis, EUiptocephala, Cal- 

 lavia, Holmia, Wanneria, Pcedeumias, Olenellus, Peachella,, and 

 Olenelloides. 



OBSERVATIONS— DEVELOPMENT 



Cephalon. — The youngest stage of growth known to me is the 

 Protaspis stage of EUiptocephala asaphoidcs [pi. 25, fig. 9]. In this 

 the palpebral ridges are continuous with the transversely elongated 

 anterior lobe of the glabella and arch about the spaces between the 

 glabella and the eye lobe, and continue back across the posterior 

 border of the cephalon. The segmentation of the cephalon is beauti- 

 fully shown by figs. 9, 10, and 22, pi. 25. In figs. 9 and 10 the 

 occipital segment merges into the strong ridge and spine formed by 

 the next two anterior segments ; the fourth anterior segment curves 



^ The genus Mesonacis is more typical of the family than the genus Olenellus 

 and as Mcsonacidse was first used, I shall continue it in this paper. 



