GASTKIUCEKAS. 83 



ribs, as G.(/lobulosio)i Meek and Wortlieu, wliilc souk; species of (Hyphioceras 

 have umbilical i-ibs and, in their youth, also the elliptical cross section, as 

 Glyph ioccras diudeiiia Goldt'uss. But the two extremes are widely separated 

 from each other, as GastrioccrasjossfeYevneuW and Gontafitcs sphrericiis Martin. 

 This genus has been looked upon by 8teinmaiui as the jincestor of the 

 trachyostracan families of the Trias, the Ceratitida; and the I'ropitida'. Dr. 

 K. von Zittel agrees with this opinion as to the origin of tlie Tropitida-, but 

 thinks the Ceratitidje developed out of the Prolecanitida^,, which is in agree- 

 ment with the writer's studies on the development of Ceratites of the Trias. 



Gastrio(;ebas branneri Smitli. 



PI. XI, tigs. 8-13. 



180(i. Gastriocerm hraruwri., J. F. >Smith, Proc Am. I'liilos. Soc, Vol. XXXV, 

 p. 257. PI. XXIII, figs. 1-6. 



The adult shell is discoidal, with low, narrow whorls of semilunular 

 cross section; the adult whorl is very evolute, embracing not more than a 

 third of the preceding one, and the increase in height and breadth is 

 extremely slow\ The young whorls are ]iroportionally broader and more 

 involute, so that tlie innbilicus of the j-ounger part of the shell is deeper, 

 but widens rapidly with age, as the involution decreases. G. hranneri is 

 the most evolute species of Gastrioceras known in the Carboniferous, and 

 approaches the narrow evolute Permian type, described by Gemmellaro 

 from Sicily: but the Sicilian form still retains the strong constrictions, 

 and has also acquired the spiral striae that are characteristic of Permian 

 Gastrioceras. 



Dimensions. 



Millimeters. 



Diameter .39. 5 



Height of last whorl 10. 5 



Width of umbilii'iis 19. 



Breadth 15.0 



Height of last whorl from top of preceding S. 



The specimen shows nine whorls at the diameter of 39 5 mm. 



Sutures. — The sutures consists of three external lobes and as many 

 saddles. The siphonal lobes are long, narrow^, and pointed; the first lateral 

 broadly pointed, and on the umbilical shoulder is another shallow lobe, 

 broad and pointed. The siphonal saddle is narrow, with the usual indenta- 

 tion at the end; tlie first lateral saddle is broadly rounded and deep, the 

 second lateral saddle shallow and inclined to be pointed. The inner lobes 



