THE PALEONTOLOGY OF MINNESOTA. 



LSalpingostoma. 



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 lunulse, 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 striae, 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 a rule 

 the transverse lines predominate in the umbilical regions, and occasionally also 



