492 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Professor McCoy's section Temnochilus was originally made by him to 

 include species belonging to the Trematoditscus group ; but as the shells of the 

 latter section differ materially in form and ornamentation from some of the 

 other species included by him, such as his N. coronatus (the first of his 

 iigured species), it seems desirable to restrict the name Temnochilus to that 

 and allied species. Consequently, Professor Worthen and the writer pro- 

 posed to separate the peculiar Carboniferous group of discoid species with 

 a wide, shallow umbilicus, narrow, non-embracing, generally more or less 

 rounded or oval volutions, ornamented with distinct revolving carinas and 

 furrows, under- the name Trematodiscus, which name was suggested by the 

 fact of most of the species having the umbilicus perforated in the middle. 

 This character, however, sometimes occurs in other ancient types of Nautili, 

 and cannot be considered peculiarly characteristic of this section. Still, as 

 these shells form a rather marked group, apparently confined to the Carbon- 

 iferous rocks, it is certainly desirable, from a palaeontological point of view, 

 to range them as a distinct section. Gr. B. Sowerby, jr., figured, in his Con- 

 chological Manual (fig. 475), a typical Trematodiscus under the name Sim- 

 ]degas (intended for Simplegades, Montfort). This, however, was inadmis- 

 sible, because Montfort's type of his Simplegades, as he wrote it, belongs to 

 some section of the old genus Ammonites, and could not, be retained for a 

 Nautiloid type, although Blainville had improperly proposed to do so. 



Some time back, Professor Worthen and the writer proposed, as a sub- 

 generic name, Endolobus, for a large Nautilus not differing very materially 

 in form from species that seem to fall into the group Temnochilus as here 

 understood ; but which is provided with a kind of lobe, or flexure, of the septa 

 on the inner side. Having since observed, however, that this character occurs 

 in other species, which, upon all other characters, would fall into several of 

 the different sections, I do not believe now that the section Endolobus can 

 be sustained; and, even if it could, Montfort's name Bisiphites would prob- 

 ably have to be retained for it. 



The group for which Professor Worthen and the writer used the name 

 Solenochilus is almost certainly the same for which d'Orbigny proposed the 



siphon is removed from the peripheral margin as in the Nautiloid forms. Still it is so near the outer 

 margin that it would only have to he placed about two or three times its own breadth farther out, 

 to bring it against the periphery, and thus make the shell a Goniatite, as that genus is usually under- 

 stood. Such examples show how gradually the distinctions between genera and larger groups are 

 found to fade away as our knowledge of the intermediate types extends. 



