OLENELLUS AND OTHER GENERA OF MESONACID.^ 263 





Fig. 17. 



Fig. 16. 



Mesonacis mickuntsi (Schmidt). 



Fig. 16. Cephalon, copied from Schmidt, 1889, fig. i, p. 193. 

 17. Pygidium, copied from Schmidt, 1889, fig. 9, p. 193. 



This species is known only by fragments that have been most 

 fully described and illustrated by Schmidt [1888 and 1889]. Dr. 

 Schmidt's restoration of the posterior portions of the thorax is copied 

 on pi. 26, fig. 4. The similarity to Mesonacis vermontana is shown 

 by figs. I and 2, pi. 26. Schmidt [1889] says that the pygidium 

 shows a slight notch on the back edge. He also observed traces of 

 transverse furrows on the axis and very faintly on the lateral lobes. 

 The fragments of the hypostoma indicate that the general form is 

 similar to that of Olenellus. 



The cephalon of M. mickzvitzi is much like that of M. vermontana, 

 and the pygidium, posterior thoracic segments, and the great spine 

 on the sixth ( ?) segment from the pygidium are also of the same 

 type. Dr. Moberg [1899, footnote, p. 319] thinks that as the generic 

 name Schmidtia is already in existence, and that as the type species 

 is so imperfectly known it would be well to retain Schmidtia for it, 

 and not refer it to Mesonacis. These reasons do not appeal strongly 

 to me, and as Schmidtia was preoccupied by Volborth in i860, I 

 think it is best to refer the species to Mesonacis and retain it there 

 until further information of it is obtained. Dr. Moberg [1906] 

 proposed Schmidtielhis to take the place of Schmidtia, but, as stated 

 above, I think it best to retain the species under Mesonacis until 

 more is known of it. 



Formation and Locality. — Lower Cambrian : The following 

 localities are given by Schmidt, 1888, p. 19: (i) lower layers of the 

 Fucoid sandstone^ on Jaggowal Brook: (2) at the same horizon 

 near the cement works on Kunda Brook; and (3) glauconitic sand- 

 stone at the base of the section in Streitberg ; all near Reval, Govern- 

 ment of Esthonia, Russia. 



^ Esthonia formation of jNIarcou [1890. pp. 360-361L 



