424 On the Palceozoic Bivalved Entomostraca. 



24. Primitia semicircularis, nov. PL XIII. figs. 10 a, b, c. 



Length .- T j height -i-i, thickness T ' T inch (8:6:4). 



Carapace-valves moderately convex, subovatc, sometimes sub- 

 orbicular, with a straight back, and with one end more or less 

 acute at the dorsal angle, and the other well rounded and form- 

 ing a bold semicircular curve with the ventral line. The ventral 

 margin is thickened, as in P. ovata. There is no doubt that 

 P. semicircularis, P. ovata, P. ubsoleta, and P. oblonga are all 

 closely allied forms of the simplest of Primitia' ; still, presuming 

 that their soft parts may have had distinctive characters (as is 

 likely) we keep them apart for convenience. They are grega- 

 rious, and, together with P. mundula, form a considerable pro- 

 portion of some parts of the Upper Silurian Limestone of 

 Sweden. 



In the Scandinavian limestone (drifted), North Germany. 

 P. pusilla seems to represent P. semicircularis in Britain. 



25. Primitia pusilla, nov. PI. XIII. figs. 11 a, 116. 



Length yuoo> height r |* inch. 



This little smooth subovate form may possibly be a variety of 

 P. semicircularis ; but its less convexity and more rounded out- 

 line distinguish it, to say nothing of its faint sulcus — a feature, 

 however, which may possibly be present in some specimens of 

 P. semicircularis. From the Wenlock Limestone, near West 

 Malvern. 



26. Primitia concinna, Jones. 



Cytheropsis concinna, Ann. Nat. Hist. ser. 3. vol. i. p. 2-19, pi. 10. 

 figs. 3,4. 



This neat subcylindrical carapace probably belongs to a furrow- 

 less Primitia ; there is, indeed, a faint dorsal hollow in the 

 original, which may stand for the sulcus. From the Lower 

 Silurian Limestone on the Ottawa River, Canada. 



The same form is not uncommon among the little Entomo- 

 straca of the Upper Silurian Limestone of Gothland (from 

 Dr. Lindstroni) ; and we have seen one like it also in the Upper 

 Silurian Limestone of Arisaig, Nova Scotia (from Dr.Honeyman). 

 It is not impossible that Eichwald's Leperditia minuta may prove 

 to be the same species; for though the published figures* are 

 not quite similar, yet some specimens with which M. E. d'Eich- 

 wald has favoured us can scarcely be distinct ; the state of pre- 

 servation, however, hinders exact comparison. 



* Bullet. Soc. Nat. Imp. Moscou, 1S54, part 1. p. 99, pi. 2. fig. 6; and 

 Lctha:a Rossica, livr. 7 (1860), p. 1335, pi. 52. fig. 2. Schrenk finds L. 

 minuta in the Lower Silurian of the Baltic Provinces (Untersueh. p. 195). 



