GASTROPODA. 837 
Palewacmea humilis.] 
Lindstrém places this genus with Tecturide (Acmeide, Carpenter) and perhaps 
justly, but, as the family is not'recogmized by some conchologists and as it is highly 
probable that the Paleozoic patelloid shells are more intimately related among 
themselves than to recent types, it has seemed to us the wisest to embrace them all 
provisionally in one broad family, the Patellide. 
PALHACMHA HUMILIS, ”. Sp. 
» 
PLATE LXI, FIGS. 45—48. 
Shell depressed conical, rather small, broadly subovate, the anterior part of the 
outline semicircular, the posterior slightly prolonged and more narrowly rounded 
centrally; apex pointed, not quite erect, leaning slightly forward and situated a short 
distance in front of the center. Surface marked with obscure, fine, concentric lines 
and usually with several strong wrinkles or undulations of growth. The latter are 
more or less variable and irregular, and in some cases may be wanting. On casts 
of the interior they are very faint or quite indistinguishable. Muscular scars not 
well preserved by any of the specimens seen, apparently detached and forming 
an oval band about the apical third of the cast. Length (small specimen) 12 mm.; 
width 10.7 mm.; hight 4.5 mm. In a larger specimen these dimensions are 
respectively 15.5, 14 and 5 mm. 
The absence of radial surface markings will distinguish this species from the 
following forms of Scenella, and the subcentral position of the apex renders equally 
good service in separating it from the preceding species of Archinacella. As to 
Paleacmea, it is certainly distinct from all other forms known to belong to the genus. 
Formation and locality.—Stones River group, Vanuxemia bed, Minneapolis; Black River group 
Ctenodonta bed, at several localities in Goodhue connty, Minnesota. 
Oollections.—Geological and Natural History Survey of Minnesota; EK. O. Ulrich. 
Museum Register, No. 5104. 
Genus SCENELLA, Billings. 
Scenella, BILLINGS, 1872, Can. Nat. and Geol., vol. vi, p. 479; and Pal. Foss., vol. ii, p. 77. 
For generic characters see page 822. 
This genus has never been properly defined and it is perhaps impossible to do so 
even to-day. Still, we are convinced that, with S. reticulata Billings as the type, it 
may be made to include an easily recognizable and thoroughly natural group of 
patelloid shells having relations to Palwacmea on the one side and to Stenotheca on 
‘the other. As understood by us the genus is characterized by the subcentral apex 
and the radial strie. The latter vary in strength, being sometimes rather coarse, 
but in most cases very fine. When the radii are coarse (S. radialis) they show 
