GASTK"!'. >l>\ 



direction of the curve occur- is usually marked by a Alight seam-like line 

 or interrupt ion. In /,'</.// I.V/WM mi ;nnl Knnmpli<ilpUni9 the sigmoid character 

 of the curve is always distinguishable but there is no intermediate interruption. 



A tnu> >lit-kmd does not occur in any of the Ripkutomida, yet it would be quite 

 reasonable to consider the angular or lamellar periphery of the whorls as its 

 representative. Or it may be that the slightly sinuate outer parts of the lines of 

 growth on the upper side of the whorls are homologous with the lunulte crossing 

 the band of true /'/--I, ><>(,, mm -H-ln . Still, the evidence at hand is insufficient to establish 

 oit her view. Kor the present then we have two well marked differences between 

 the shells of the two families that may be utilized even should the suggested 

 limnologies admit of demonstration. The first of these differences lies in the 

 absence of an apertural -lit ami the resulting slit-band, while the second 

 may be expressed by saying that the retral sweep of the lines of growth in 

 the Plturotoinariitio is never diminished but, on the contrary, is nearly always 

 im-MM-fl just before reaching the band, while in the Raphistomidtr it is decreased 

 and often overcome entirely on the peripheral carina. In cases like StMMptefopfenM 

 -//;/(/-. in which the carina is extremely developed, the second curve is again 

 overcome by a third which is strongly retral. The last, it seems to us, has no 

 further significance than the preservation from injury of the anterior outer angle 

 of the carina which would have been exceedingly liable to breakage if the second 

 curve had continued. 



We place here in all five genera. The position of the first three we regard as 

 unquestionable, the fourth, Omospira, differs very decidedly from the rest in the 

 rounded form of its volutions and relatively high spire, while the fifth, Sen lit- 

 doubtful because there is yet much to learn respecting its most important charac- 

 teristics. If the last should prove to have the really essential features of the family 

 then its systematic position would most likely be between Kaphistoma and 0mo.<y/iV./. 

 All of these genera have heretofore been placed with the Pleurotomariidcr. For general 

 remarks on them, farther than those about to follow, the reader is referred to our 

 discussion of that family on a subsequent page. 



II'M-HISTOMV. Hall. 1847.* Shell sublenticular or plano-convex, the spire flat, 

 the sutures close; volutions triangular in section, sharply angular and generally 

 thin at the periphery; there is neither a slit nor a band; umbilicus varying in size 

 but nearly always present; aperture turned backward .-lightly so as to form a 

 shallow notch at the outer angle; lines of growth only; on the flattened upper surface 

 these are slightly sigmoid and usually interrupted by a raised line between the two 



Pl. Nw York. vol. I. p. * 



