88 BULLETIN OF THE 



PHYMATOID^.* 



Phymatoceras Hyatt-I 



Abdomen may be flattened or rounded, but never acute ; has no chan- 

 nels in the adult. Envelopment covers the abdomen of each internal whorl. 

 Radii of the spiral increase more slowly than in the succeeding genera. 

 The young are smooth on the first or second whorl, the tubercles begin 

 either on the second or third whorl, and, gradually dividing, spread them- 

 selves out upon the abdomen as bifurcated pilse, which disappear on the 

 borders of the channels. The keel makes its appearance at an early stage, 

 probably on the second whorl, but the channels are not visible until a much 

 later period, and disappear in the adult. 



Abdominal lobe broad and deep. Superior lateral broader, but of about 

 the same depth ; inferior lateral, very shallow. Superior and inferior lat- 

 eral cells equally divided ; both are short, broad, and but slightly indented 

 by the minor lobes. 



Phymatoceras robustum Hyatt. 



Loc. Plateau de Larzac ; Coll. Dr. Krantz. 



The abdomen of this species is flattened, the sides of the whorls gibbous 

 and narrow, and the keel very prominent. The channels in the young are 

 shallow, and the whorls unusually broad. Superior lateral cell is deeper 

 than the inferior lateral, and the latter is straight ; the auxiliary cell is 

 divided by one small auxiliary lobe. 



Hammatoceras J Hyatt. 



Abdomen may be either rounded or acute, always keeled, but never 

 sulcated. Pilae are prominent and straight. Envelopment may extend 

 over one half the sides, or only cover the abdomen of each internal whorl. 

 The young develop as in Phymatoceras, but are generally much broader; 

 the pilrc, also, do not become prominent so soon. Nor do they invariably 

 begin by the development of tubercles on the sides, but may make their 

 appearance as fine, raised lines, and afterwards become tuberculated. 



During the earlier stage of growth the different species have a very close 

 resemblance to the adult Macrocephali. The lobes are more complicated 

 than in Phymatoceras. Abdominal lobe broad and deep, and continued 

 into two long, narrow, minor lobes. Superior lateral narrower than the 

 abdominal. Inferior lateral hardly wider than the minor lobes of the 

 superior lateral, and of about the same depth. Abdominal cell blunt. 

 Superior lateral and inferior lateral very narrow and deeply indented by 

 the minor lobes. 



* Includes part of the I'alciferi. t *C/xa, a swelling. J Afi/xa, n knot. 



