196 GENESIS OF THE AEIETID.E. 



the breadth or thinness of the whorls. Some of these have stouter adult whorls 

 than usual. If the slight siphonal ridge were absent, there would be a closer 

 resemblance in form to Psil. phndrbe, var. ervgatum. The young have at first 

 deep umbilici, due to the abrupt umbilical shoulders, Plate VIII. Fig. 10, 13, 14, 

 common to the Goniatitinula and early naepionic stages of the Arietida?. This 

 stage is succeeded by flatter whorls and less abrupt umbilical shoulders, which 

 last in some cases throughout the first three whorls, but the fourth whorl is apt 

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

 the umbilicus when the fourth whorl is completed is thus altered in such speci- 

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

 species after the earlier goniatitic proportions are outgrown. In some specimens 

 this change takes place much earlier. The pilae are introduced generally after 

 the second stage of growth, and but very rarely before the reduction in the 

 comparative breadth of the whorl begins. The sutures are also immature on 

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

 laterals, as among true Arietida?. The other lobes are pointed, and the saddles 

 serrated. 



The Museum at Semur and the Museum of Comparative Zoology afford 

 ample material for tracing the connections between this species and Agas. striaries. 

 Quenstedt, in "Ammoniten des Schwabischen Jura," p. 106, has also noted the 

 affinities of this species and striaries. According to Oppel, it appears in the bed 

 immediately above the Bucklandi bed, and in the Museum of Stuttgardt is a 

 specimen in the Geometricus bed from Degerloch. In England, however, it is 

 usually found associated with Deroceras planicosta in the Obtusus zone, and at 

 Semur with Schlot. angulata, and above this in the Geometricus bed. The speci- 

 mens described and figured by Hauer and Geyer from the Hierlatz fauna seem 

 to be unquestionable, since no. other species with which we are acquainted has 

 the peculiarities of this form. 



Agassiceras striaries, Hyatt. 



Plate IX. Fig. 14, 15. Summ. PI. XIII. Fig. 6. 



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



Amm. striaries, Quenst., Amm. Schwab. Jura, p. 105, pi. xiii. fig. 24-26. 



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



Locality. — Semur. 



Sides of the whorls in adult specimens may be either flattened or decidedly 

 gibbous, and also either plicated or smooth. Abdomen is very broad, depressed, 

 convex, smooth or very slightly ridged by lines of growth. The position of the 

 siphon is often indicated by a ridge. The young are smooth for the first three 

 whorls, the plications begin to appear on the fourth whorl. These observations 

 were made upon five specimens in the Museum of Comparative Zoology, which 

 had been labelled Amm. planorbis by M. Boucault ; but they differ from that spe- 

 cies in the smaller size and greater proportional bulk of the whorls, the breadth 

 and depressed convex form of the abdomen, and the presence of a siphonal ridge. 



