826 THE PALEONTOLOGY OF MINNESOTA. 



[Tryblidiura modestum. 



usual, curve around in front to meet beneath the apex. (See fig. Ic, p. 820.) 

 Lindstrom probably overlooked these anterior pairs because he noticed a slightly 

 elevated, transverse, medially disconnected band just in front of the sixth pair. But 

 this band is really the posterior boundary of the scars of an umbonal pair* of 

 muscles which we have shown to exist in a number, if not all, of the Paleozoic 

 PatellidcK. 



As already stated, we regard T. conicum Whitfield, as a Scenella, while Metoptoma 

 simplex Billings, which Whitfleld places under Tryblidium, and T. piliolum Whitfield 

 probably belong to Archinacella. T. ? acutum Whitfield, with its flattened area-like 

 slope beneath the projecting apex, seems to us to indicate an undescribed generic 

 type. 



TRYBLIDIUM MODESTUM, . sp. 



PLATE LXXXIl, FIGS. 1 and 2. 



Shell small, somewhat acutely ovate in outline, the apex being pointed, scarcely 

 incurved and projectly slightly beyond the narrowly rounded anterior margin of 

 the aperture; the latter is scarcely, if at all, arched. Surface not well preserved, 

 apparently marked by faint lines of growth. Internal characters unknown. Length 

 10.3 mm.; width 8.5 mm.; hight of apex about 2 mm.; greatest hight (near center of 

 length) about 2.8 mm. 



This species is founded upon a single imperfect shell. As the muscular scars 

 have not been observed the generic reference is perhaps a little doubtful. Still, the 

 form of the shell is such that we are fairly confident that it will prove to be a true 

 Tryblidium. Of described species T. erato Billings sp., from the Black Kiver lime- 

 stone of Canada, appears to be the nearest. On comparison with the description of 

 that species (it has not yet been figured) it is evident that T. modestum is much 

 smaller, relatively wider posteriorly and not so convex. The fact that the shell is 

 narrower in front than behind will distinguish it at once from all the species of 

 Archinacella. Otherwise it looks very much like A. patelliformis. It also resembles 

 Stenotkeca unguiformis, but is wider and the beak is not so prominent in front, while 

 the surface markings are quite different. 



Formation and locality. Black River group, Ctenodonta bed, near Cannon Falls, Minnesota. 

 Collection. E. O. Ulrich. 



Oenus HELCIONOPSIS, n. gen. 



For generic description see page 821. 



Of this genus we have at present only three species, the two about to be described 

 and an Upper Silurian one from Gotland, which Lindstrom placed doubtfully 



These umbonal scare are shown In fig. 37. pi. i, of LlndstrCni's "Gastropoda and Pteropcda of Gotland," 



