196 GENESIS OF THE ARIETIDZ. 
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 Psi/. planorbe, var. erugatum. The young have at first 
deep umbilici, due to the abrupt umbilical shoulders, Plate VIII. Fig. 10, 13, 14, i 
common to the Goniatitinula and early nzepionic stages of the Arietide. 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 pile 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 
i laterals, as among true Arietidz, The other lobes are pointed, and the saddles 
serrated. 
The Museum at Semur and the Museum of Comparative Zodlogy afford 
ample material for tracing the connections between this species and Agas. striaries. 
Quenstedt, in “ Ammoniten des Schwibischen 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 Schiot, 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, Hyarr. 
Plate IX. Fig. 14, 15. Summ. Pl. XIII, Fig. 6. 
Amm. striaries, Quenst., Der Jura, p. (0; pl. vill, figs b, 
Amm. striaries, Quenst., Amm. Schwiib. Jura, p. 105, pl. xiii. fig. 24-26. 
Psiloceras planilaterale, Hyarr, Bull. Mus. Comp. Zool., 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 t 
were made upon five specimens in the Museum of Comparative Zodlogy, 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. 
