1874.] 227 [Hyatt. 



young umbilicus. The umbilical edges become reduced in course of 

 growth, and also, as explained before, by the comparative reduction 

 of the rate of growth of the transverse diameter of the whorl ; but 

 this only endures for a time, and the fourth whorl again increases fast 

 enough to be somewhat broader than the third. The aspect of the 

 umbilicus when the fourth whorl is completed is thus altered from 

 deep to shallow, just as it changes at much earlier stages in other 

 species, after the earlier goniatitic proportions are outgrown. 



The pilse are introduced generally after the second stage of growth 

 begins and but very rarely before the reduction in the comparative 

 breadth of the whorl begins. The septa are also immature on the 

 fourth whorl, but the abdominal lobe is considerably deeper than the 

 superior laterals. The other lobes are pointed and the cells serrated. 

 This species, according to Oppel, appears in the bed immediately 

 above the Bucklandi bed and in the Museum of Stuttgart is a speci- 

 men in the Geometricus bed from Degerloch. In England, however, 

 it is usually found associated with Deroceras planicosta, and at Semur 

 with JEgoc. angulatum. They also have short living chambers and 

 septa whidh do not permit of their being joined with Psil. planorbe. 

 The Museum at Semur and the Museum of Comp. Zoology afford 

 ample materials for tracing the connection between this species and 

 striaries, there being many intermediate forms closely connecting 

 the two. 



The constant identification of this species with Psiloceras planorbe 

 is a mistake very naturally made by those who are ignorant of its 

 precise geological position, as I was at the time of the description of 

 the variety planilaterale. They seem, however, to have but little in 

 common, except the general aspect of the shell. 



Agassiceras striaries Hyatt. 



Amm. striaries Quenst., Der Jura., p. 70, pi. 8, fig. 5. 



Psiloceras planilaterale Hyatt, Bull. Mus. Comp. Zool., No. 5, p. 73. 



Loc. Semur. Coll. Boucault. 



Sides of the whorls in some adult specimens flattened but in others 

 decidedly gibbous ; they may be either plicated or smooth. Abdomen 

 very broad, depressed, convex, smooth or very lightly ridged where 

 the thickened lip of the rostrum marks the limit of the annual growth 

 of the shell. The position of the siphon is often indicated by a 

 raised line. The young are smooth for the first three whorls, the pli- 

 cations beginning to appear on the fourth whorl. 



Remarks. The observations were made upon five specimens which 

 were labelled Amm. planorbis by M. Boucault, but they differ from 



