\ 



these appear to he within tin- limit* <>f the yenn*. 1'rof .lone* has also referred 

 several l.owerand I'pper Silurian >pecie* t> the p-mi.*. l-ut here, it *oems to me 

 generic relation* arc n >'a*t <linilttfiil. The following tsso *pene* .it 



any rate, an- almo*t certainly not ( '///' , ct they mu.*t In- placed here because 



their known characters are more in accordance with tlii- ^emi* tlian with any of 

 thi others that have \- Ii-h."l. 



At pre*ent the principal diagnostic feature of < ';/////-,//</. that i>, \\ith the 

 palcontdlii^ ic ral-lteted edge of the ri^ht valve. This peculiarity, if my 



memory i- not at fault, has not yet been shown to exi-4 in any of the Silurian 



ics hitherto referred to the ^'cnu-. It does however exist, and very stron^y 

 !i an iimlescril.c'l -pccic- from the lower beds of the Cincinnati group. 

 In this specie-, ho. unless all the specimens seen (about twenty) are of one 



valve only, the edges of l>oth valves are about equally grooved. 



I-'..r remark* on f 'ytlie relit : n<i. .loues and Holl, see under Hyllioci/pris and B. 

 cifliniln 



MIA. ;i. sp. 



PLA O. 



'K. Length 0.5 mm.; hlghl 0.4 . nun. 



This species is founded upon a single carapace attached to the surface of a 

 fragment of the /.oariuin <tf I'n<lii/Jicty<i foliata. The smaller valve is exposed to 

 view, ami around it the overlapping edge of the larger, presumably the right valve, 

 i* di>tin< tly defined. The outline is broad-oval, almost circular, and as near as can 

 l>e determined, the surface of the smaller valve is moderately and quite uniformly 

 convex, and exhibits neither a central depression, a tubercle, nor markings of any 

 kind. The specimen was found in association with valves described on a preceding 

 page as >VA//i iiltrlln? sul>rutund. They are distinguished by a small, subcei 

 tubercle, but as they have the same rounded outline, it is possible that a better 

 preserved series of specimens may show them all to belong to one species. That the 

 synonomy may. in case tin lity is converted into a fact, l>e simplified, I have 



used the same specific name for both. 



Formation and locality. Liw<-r th : .ton shales, Minneapolis Minnesota. 



