G94 PROCEEDIXGS OF THE NATIONAL MUSEUM. voi..xxiii. 



lar sinus; they are elong-ate and rather laro'e; in the dorsal valve they 

 are elong-ate, with the major axis inclining- forward toward the median 

 line. The middle and outside laterals are situated in the trapezoidal 

 area of the ventral valve, but neither is clearly separal)le from the 

 other or from the central scars. In the dorsal valve the position of 

 "the middle and outside laterals is shown, but not their form or size. 

 The transmedian sears in the ventral ^ah'e are seen just back of the 

 anterior laterals, 1)ut they have not been ol)served in the dorsal valve, 

 owing to the imperfections of the shell. 



()hservat!o)if<. — This appears to be a representative of (K (Z.) iKumi- 

 natm^ which is so abundant in the Middle Cambrian of the Upper Mis- 

 sissippi Valle}" and the passage l)eds l)etween the Caml)rian and Ordo- 

 vician adjoining the Adirondack Mountains of New York. It differs 

 from that specdes in its greater average Avidth and in its surface char- 

 acters. 



Mr. Matthew's illustrations are diagrannnatic and drawn from com- 

 pressed, impei-fect material. With a large series of well-preserved 

 specimens T iind that the species does not depart materially from the 

 typical O. {LuKjulej)!^) (icxm hiatus. ]\Ir. ^Matthew referred the species 

 to a pre-Cand)rian famia, but in collections made by Mr. S. Ward 

 Loper it occurs on slabs of siliceous shale and sandstone associated 

 with heads and fragments of Paradoxides. 



L/iujiilrlhr til III /(la Matthew is founded on a longitudinally com- 

 press(Kl and distorted ventral valve of this species. A number of such 

 in various stages of transition between the two forms occur in the 

 U. S. National Museum collections. 



Li'ptdhohM atavuH Matthew appears to l)e founded on the 3"oimg- of 

 O. (Z.) gregvxL There is a transition in form between the tvpes of 

 Leptoholus atarn.^ and the nndoubt(Ml forms of (K (Z.) (jregioa. 



This is one of the most interesting- species I have seen. 15y its 

 coarsely pitted inner surface it recalls O. {Lingalella) (hivisi of Eng- 

 land, and (K {Luigiilrlla) irilUs! of tht^, southern Appalachians. The 

 elongate ventral valve is like that of O. {LuiguJrqi/x) acuin hiatus, while 

 the interior scars and markings are those of ()l)olus. 



Foi'iiiatioii ami locality. — Middle Caml)rian. Paradoxides beds. 

 Siliceous shale and thin-l)edded sandstones west side of ]McLean P)rooiv. 

 above Marion bridge road, Salmon Hlver, (lillis Hill. 13 miles south 

 of Marion l)ridge, Cape Breton Island, Nova Scotia. 



Subgenus ACRITIS. 

 OBOLUS (ACRITIS'.') RUGATUS, new species. 



The concentric surface lines which ari> the characteristic feature of 

 this shell are of the same type as those of <>. {Acritis) aiifiquwsriinis^ 

 although nmch coarser, andare prominenton the postero-lateral margins 

 in much the same manner. It is a very rare form, only one s})ecimen 



