898 THE PALEONTOLOGY OF MINNESOTA. 
~ [Salpingostoma. 
apertural sinus in all Bellerophontacea having a slit-band. In some it was very long 
(Bucania and Conradella), but in the majority it was comparatively short; and in all 
cases it is the feature that gave rise to the slit-band, the posterior end of the slit 
having been pushed forward in proportion with the growth of the shell. 
In only two genera of the suborder, however, Salpingostoma and Tremanotus, 
was the slit closed in front. In both of these genera the aperture is enormously 
expanded, not only laterally and ventrally, but dorsally as well, and it is the last 
peculiarity, one in which these genera stand practically alone, that we consider as 
the most important. The anterior closing of the slit was, we think, merely inci- 
dental to this expansion of the aperture, and perhaps dominated entirely by the 
necessity of overcoming the extreme liability to fracture to which the aperture 
would have been subject had the slit been allowed to continue to its outer 
margin. : 
Regarding the dorsal slit of Salpingostoma, it is questionable if the entire length 
of it that is represented by a ridge on casts of the interior was open. There is some 
reason to believe that a portion of the posterior end was covered by a thin film of 
shell. The greater portion of it, however, seems to have been permanently open. 
While there may be some doubt about the covering of the posterior end, there is 
none when we come to the anterior end. Here, from the beginning of the apertural 
expansion, backward for a distance equalling about one-half of the transverse 
diameter of the volution, the slit is undoubtedly closed, though continuing in some 
specimens as a gradually diminishing furrow on the inner side of the shell. In 
other specimens, of the same species even, there is a broad internal thickening, 
leaving a furrow instead of a ridge on casts of the interior. Behind the slit there is 
a distinct band with lunule, precisely as in Bucania and Bellerophon. 
The surface markings of Salpingostoma are practically the same as in Bucania. 
Beneath the apertural expansion they consist of more or less oblique and wavy, 
wrinkled, revolving striz, interrupted at subregular intervals by lines of growth. 
The former may be represented, as in S. buelli, by small, partially disconnected 
knots, which are arranged in series in such a manner that when viewed with the 
light coming from different directions the predominent element of the markings is 
changed from the oblique to the longitudinal. In S. sculptilis the revolving lines 
are zigzag and unite with the transverse lines in producing a network in which the 
pattern is complicated by an extra thread running obliquely through the alternating 
meshes. In nearing the mouth and continuing over the expansion the revolving 
lines usually become the most conspicuous element of the surface ornamentation, 
but they seem never to lose their irregularly wavy character entirely. As arulé 
the transverse lines predominate in the umbilical regions, and occasionally also 
