1874.] 21 [Hyatt. 



the pilse again join, but this is not so frequent as has been supposed. 

 They more often remain separated until old age. 



The early occurrence of this form in the Planorbisbed is estab- 

 lished by repeated observations on the part of Profs. Quenstedt and 

 Fraas in Wurtemburg. The separation of the pilse is not uncommon 

 in other groups, especially in Perisphinctes. The original of Amm. 

 angulatus Sow., which I saw in the British Museum, is only a mal- 

 formed communis. 



JEgoeeras Charmassei Hyatt. 



Amm. Charmassei D'Orb., Terr. Jurass., Ceph., p. 296, pi. 91, 92. 



Besides the characteristics mentioned in the description of JEgoce- 

 ras angulatus the following may be added. On the sixth volution, 

 the extremely gibbous form of the young begins to change. The 

 whorl increases more rapidly, the abdomen is narrower, and the 

 pilse as in preceding species, with this exception. On this volution, 

 or perhaps on the fifth, they become bifurcated, or else have inter- 

 mediate short pilse interspersed between the longer ones. The septa 

 have remarkably large abdominal lobes, shallower than the superior 

 laterals, but with a much more ragged outline. The siphonal 

 cell is extraordinary in this respect. It is very large, and marked 

 with several lateral minor lobes and cells. The remaining lobes and 

 cells are much more complicated than in angulatus. 



On the sixth volution the form of the whorl changes exactly as in 

 angulatus. The envelopment of this whorl equals one-half of the side 

 of the sixth, whereas in angulatus the envelopment does not equal 

 this until it reaches the ninth volution. The envelopment at the 

 same age in this species, that is on the ninth whorl, covers full two- 

 thirds of the side of the eighth whorl. There is a form in Prof. 

 Fraas' collection from Mohringen answering to the young of Char- 

 massei, as figured by D'Orbigny, pi. 91, and another from Filder, 

 which is precisely intermediate in its characteristics between this and 

 the smoother, flatter variety figured on pi. 92. The oldest specimens 

 in the possession of the Museum of Stuttgart measured 53 mm., and 

 the last whorl 23 mm. A. angulatum parts with its pilse and grows 

 smooth much earlier apparently than A . Charmassei. Probably this 

 occurs at about the same age, but the superior size of Charmassei 

 makes it seem older when the old age characteristics begin to appear. 



