160 THE DEVONIAN OF MISSOURI. 



ventral side. The tube gradually enlarges back of the aperture, and suddenly expands 

 to the point of the greatest transverse section. The crenulated band in the cast forms a 

 deep, narrow fossa at the base of the grand chamber, and is crossed by furrows, of which 

 there are about four in the space of twelve mm. 



Aperture sub-oval, having a length equal to the greatest ventro-dorsal diameter, 

 or about twice its transverse diameter. The small aperture is indicated by a sinus in the 

 margins of the large aperture. 



Air-chambers numerous, regularly increasing in depth from the apex to near the 

 base of the outer chamber, varying in depth from four to six mm. in the length of seventy 

 mm. The last two air-chambers are shallower than the preceding. Septa not observed. 

 Sutures straight and horizontal. 



Siphuncle, test and surface-markings unknown." 



Remarks — One Missouri specimen has many of the characters of G. impar. The 

 shell is constricted at the base of the living chamber. The outer two air-chambers are 

 much shallower than the others. The depth of the air-chambers ranges up to 7 mm. 

 rather than to 6 mm. as in the type. The shapes of the living chamber and of the aperture 

 are indistinct. 



Occurrence — Grand Tower limestone of Little Saline Creek. 



Gomphoceras missouriensis n. sp. 



Plate 37, figures 2, 3, 4 and 5; plate 38, figure 2. 



Description — The known specimens of this species are all moulds of living chambers. 

 The living chambers are small. They range in size from 18 mm. in greatest diameter 

 to 38 mm. As the outer ends are missing the length of the chamber cannot be de- 

 termined. In a specimen 18 mm. in diameter 23 mm. of length is preserved, and in the 

 specimen 38 mm. in diameter the length is 38 mm. The peculiarity of the chamber 

 is in its almost straight sides of 12 to 14 mm.; abrupt narrowing from 18 to 13 mm.; 

 and the much narrowed part proceeding outward at an angle of about 145 degrees 

 from the outer part. The aperture of the only specimen that retains it nearly complete 

 is subangular at the sides and has a diameter of 13 mm. in one direction by 10 mm. in 

 the other with regularly curving sides from the subangular parts. The siphuncle is 

 small and located subcentrally. 



One specimen, which is referred to the same species, retains four air chambers. 

 These slope apexward at an angle of 38 degrees. This specimen differs from the others 

 in the restriction of the living chamber being much more pronounced. It narrows 

 abruptly from a diameter of 31 mm. to 22 mm. In this specimen the shoulder forms 

 almost a right angle while the others slope in much more gradually. 



Occurrence — Grand Tower limestone of Little Saline Creek. 



Gomphoceras plenum? Hall 

 Plate 38, figure 3. 



1886. Gomphoceras plenum Hall, Fifth Ann. Rept. State Geologist, Expl. pi. (121a) 6, 



figs. 3, 4. 

 1888. Gomphoceras plenum Beecher, Pal. New York, supplement to vol. V, pt. 2, in 

 vol. VII, pp. 33-34, pi. 121a, figs. 3, 4. 



Beecher 1 s description — "Shell large, stout, sub-fusiform. Transverse section circu- 

 lar; point of greatest gibbosity at about the tenth septum from the chamber of habita- 

 tion. Axis nearly straight. Tube rapidly enlarging to the point of greatest transverse 

 section, more gradually contracting to the outer third of the grand chamber, and ab- 

 ruptly constricted at the aperture. 



Chamber of habitation as long as the diameter of the tube at the last septum. Aper- 

 ture obscurely trilobate. Air-chambers from 6 to 7 mm. in depth over the larger portion 

 of the shell; the last three being shallower. Sutures slightly curved. 



