i.\ -i n 



I'l. Ml \l V II..S. 

 linn ; l.i k 'lit O.i:i HUM i. *iii-ss 0.3 IIIIM. 



Valves small. short. rounded-ovate, rather uniformly convex. \\itli an obscurely 

 defined, narrow border around the end- and ventral margin; near the center a faint 

 depre--ion. and immediately lifhind it .1 -mall elevation. 



The generic position ( ,f thi- -mall i- uncertain. It is placed under 



nidfrllti chiefly liecau-e it -eem- to be related to 5. untl><inn(<i. though much 

 -horter. Its outline is almost exactly as in the Ci(fltrnll>i .' *ulirntnn<{<i of this report. 

 which was al-o found a-sociated with it. I'o ilily they 1. eh mg to one species. Still. 

 a- the type <>f the ' 'f,i has neither a central pit nor a (.order, I am for the 



pre-sent obliged to regard them a- distinct. 



mintion in,'! thlnl of thi-Tri-ntnti - nneapolls, Minnesota. 



ii- AI'AlirillTKS. Jones. 



'NBs, 1W9. Ann. and Ma. Nat. II - . . ,;. p. 386. 



apace subovate, ol>loi >mewhat rounded, with a straight hinge of 



variable length: valves >ulipqiial; ed^e> tliickened, never overlapping, often beveled 

 ore: in other cases simple, and rarely with a narrow tlattened border. Sin 



more or less convex, usually smooth, without sulcus tnKereles or lobes. 



Type: .1. ,/ l,-i,.irfsii Joii' 



Tiie al>ove definition embraces a number of species that had formerly been 



,nder ]'rinlitt. l.-m-lu'lina and Isjtenliliti. I'mm the first they are distin- 



guished 1-y the absence of a -ulcu-, from the second by the absence of the 



eye-tubercle and certain -hallow depressions behind it, and from the third by the 



nee of the eye-tubercle and the equality of their valves, there being no ventral 

 overlap. From L,^,r<lH>U>i they are separated by their equal valves, the left 



apping the right in that new genus. Finally, the new genus I'rimilirllti includes 

 some very similar carapaces, but these may be distinguished, in most cases very 

 easily, by a broad though quite undefined depression or sulcus in the rentro-dorsal 

 region. 



The species of Ajmrrli all -mall, the average length being between I.n 



mm. and \J> mm., while the largest known does not e.\ (| mm. The total 



number of those known, including several undescrihed species from <>hjo. probably 

 exceeds twenty. These are distributed almo-t equally between the Lower and 

 Upper Silurian n.ck-. though in America they are known chiefly from the Trenton 

 and Cincinnati formati' 



